NAME

perlfunc - Perl builtin functions

DESCRIPTION

The functions in this section can serve as terms in an expression. They fall into two major categories: list operators and named unary operators. These differ in their precedence relationship with a following comma. (See the precedence table in perlop.) List operators take more than one argument, while unary operators can never take more than one argument. Thus, a comma terminates the argument of a unary operator, but merely separates the arguments of a list operator. A unary operator generally provides scalar context to its argument, while a list operator may provide either scalar or list contexts for its arguments. If it does both, scalar arguments come first and list argument follow, and there can only ever be one such list argument. For instance, splice has three scalar arguments followed by a list, whereas gethostbyname has four scalar arguments.

In the syntax descriptions that follow, list operators that expect a list (and provide list context for elements of the list) are shown with LIST as an argument. Such a list may consist of any combination of scalar arguments or list values; the list values will be included in the list as if each individual element were interpolated at that point in the list, forming a longer single-dimensional list value. Commas should separate literal elements of the LIST.

Any function in the list below may be used either with or without parentheses around its arguments. (The syntax descriptions omit the parentheses.) If you use parentheses, the simple but occasionally surprising rule is this: It looks like a function, therefore it is a function, and precedence doesn't matter. Otherwise it's a list operator or unary operator, and precedence does matter. Whitespace between the function and left parenthesis doesn't count, so sometimes you need to be careful:

print 1+2+4;      # Prints 7.
print(1+2) + 4;   # Prints 3.
print (1+2)+4;    # Also prints 3!
print +(1+2)+4;   # Prints 7.
print ((1+2)+4);  # Prints 7.

If you run Perl with the use warnings pragma, it can warn you about this. For example, the third line above produces:

print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.

A few functions take no arguments at all, and therefore work as neither unary nor list operators. These include such functions as time and endpwent. For example, time+86_400 always means time() + 86_400.

For functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in scalar context by returning the undefined value, and in list context by returning the empty list.

Remember the following important rule: There is no rule that relates the behavior of an expression in list context to its behavior in scalar context, or vice versa. It might do two totally different things. Each operator and function decides which sort of value would be most appropriate to return in scalar context. Some operators return the length of the list that would have been returned in list context. Some operators return the first value in the list. Some operators return the last value in the list. Some operators return a count of successful operations. In general, they do what you want, unless you want consistency.

A named array in scalar context is quite different from what would at first glance appear to be a list in scalar context. You can't get a list like (1,2,3) into being in scalar context, because the compiler knows the context at compile time. It would generate the scalar comma operator there, not the list concatenation version of the comma. That means it was never a list to start with.

In general, functions in Perl that serve as wrappers for system calls ("syscalls") of the same name (like chown(2), fork(2), closedir(2), etc.) return true when they succeed and undef otherwise, as is usually mentioned in the descriptions below. This is different from the C interfaces, which return -1 on failure. Exceptions to this rule include wait, waitpid, and syscall. System calls also set the special $! variable on failure. Other functions do not, except accidentally.

Extension modules can also hook into the Perl parser to define new kinds of keyword-headed expression. These may look like functions, but may also look completely different. The syntax following the keyword is defined entirely by the extension. If you are an implementor, see "PL_keyword_plugin" in perlapi for the mechanism. If you are using such a module, see the module's documentation for details of the syntax that it defines.

Perl Functions by Category

Here are Perl's functions (including things that look like functions, like some keywords and named operators) arranged by category. Some functions appear in more than one place.

Functions for SCALARs or strings

chomp, chop, chr, crypt, fc, hex, index, lc, lcfirst, length, oct, ord, pack, q//, qq//, reverse, rindex, sprintf, substr, tr///, uc, ucfirst, y///

fc is available only if the "fc" feature is enabled or if it is prefixed with CORE::. The "fc" feature is enabled automatically with a use v5.16 (or higher) declaration in the current scope.

Regular expressions and pattern matching

m//, pos, qr//, quotemeta, s///, split, study

Numeric functions

abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt, srand

Functions for real @ARRAYs

each, keys, pop, push, shift, splice, unshift, values

Functions for list data

grep, join, map, qw//, reverse, sort, unpack

Functions for real %HASHes

delete, each, exists, keys, values

Input and output functions

binmode, close, closedir, dbmclose, dbmopen, die, eof, fileno, flock, format, getc, print, printf, read, readdir, readline rewinddir, say, seek, seekdir, select, syscall, sysread, sysseek, syswrite, tell, telldir, truncate, warn, write

say is available only if the "say" feature is enabled or if it is prefixed with CORE::. The "say" feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope.

Functions for fixed-length data or records

pack, read, syscall, sysread, sysseek, syswrite, unpack, vec

Functions for filehandles, files, or directories

-X, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link, lstat, mkdir, open, opendir, readlink, rename, rmdir, select, stat, symlink, sysopen, umask, unlink, utime

Keywords related to the control flow of your Perl program

break, caller, continue, die, do, dump, eval, evalbytes exit, __FILE__, goto, last, __LINE__, next, __PACKAGE__, redo, return, sub, __SUB__, wantarray

break is available only if you enable the experimental "switch" feature or use the CORE:: prefix. The "switch" feature also enables the default, given and when statements, which are documented in "Switch Statements" in perlsyn. The "switch" feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope. In Perl v5.14 and earlier, continue required the "switch" feature, like the other keywords.

evalbytes is only available with the "evalbytes" feature (see feature) or if prefixed with CORE::. __SUB__ is only available with the "current_sub" feature or if prefixed with CORE::. Both the "evalbytes" and "current_sub" features are enabled automatically with a use v5.16 (or higher) declaration in the current scope.

Keywords related to scoping

caller, import, local, my, our, package, state, use

state is available only if the "state" feature is enabled or if it is prefixed with CORE::. The "state" feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope.

Miscellaneous functions

defined, formline, lock, prototype, reset, scalar, undef

Functions for processes and process groups

alarm, exec, fork, getpgrp, getppid, getpriority, kill, pipe, qx//, readpipe, setpgrp, setpriority, sleep, system, times, wait, waitpid

Keywords related to Perl modules

do, import, no, package, require, use

Keywords related to classes and object-orientation

bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use

Low-level socket functions

accept, bind, connect, getpeername, getsockname, getsockopt, listen, recv, send, setsockopt, shutdown, socket, socketpair

System V interprocess communication functions

msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop, shmctl, shmget, shmread, shmwrite

Fetching user and group info

endgrent, endhostent, endnetent, endpwent, getgrent, getgrgid, getgrnam, getlogin, getpwent, getpwnam, getpwuid, setgrent, setpwent

Fetching network info

endprotoent, endservent, gethostbyaddr, gethostbyname, gethostent, getnetbyaddr, getnetbyname, getnetent, getprotobyname, getprotobynumber, getprotoent, getservbyname, getservbyport, getservent, sethostent, setnetent, setprotoent, setservent

Time-related functions

gmtime, localtime, time, times

Non-function keywords

and, AUTOLOAD, BEGIN, CHECK, cmp, CORE, __DATA__, default, DESTROY, else, elseif, elsif, END, __END__, eq, for, foreach, ge, given, gt, if, INIT, le, lt, ne, not, or, UNITCHECK, unless, until, when, while, x, xor

Portability

Perl was born in Unix and can therefore access all common Unix system calls. In non-Unix environments, the functionality of some Unix system calls may not be available or details of the available functionality may differ slightly. The Perl functions affected by this are:

-X, binmode, chmod, chown, chroot, crypt, dbmclose, dbmopen, dump, endgrent, endhostent, endnetent, endprotoent, endpwent, endservent, exec, fcntl, flock, fork, getgrent, getgrgid, gethostbyname, gethostent, getlogin, getnetbyaddr, getnetbyname, getnetent, getppid, getpgrp, getpriority, getprotobynumber, getprotoent, getpwent, getpwnam, getpwuid, getservbyport, getservent, getsockopt, glob, ioctl, kill, link, lstat, msgctl, msgget, msgrcv, msgsnd, open, pipe, readlink, rename, select, semctl, semget, semop, setgrent, sethostent, setnetent, setpgrp, setpriority, setprotoent, setpwent, setservent, setsockopt, shmctl, shmget, shmread, shmwrite, socket, socketpair, stat, symlink, syscall, sysopen, system, times, truncate, umask, unlink, utime, wait, waitpid

For more information about the portability of these functions, see perlport and other available platform-specific documentation.

Alphabetical Listing of Perl Functions

-X FILEHANDLE

-X EXPR

-X DIRHANDLE

-X

A file test, where X is one of the letters listed below. This unary operator takes one argument, either a filename, a filehandle, or a dirhandle, and tests the associated file to see if something is true about it. If the argument is omitted, tests $_, except for -t, which tests STDIN. Unless otherwise documented, it returns 1 for true and '' for false. If the file doesn't exist or can't be examined, it returns undef and sets $! (errno). Despite the funny names, precedence is the same as any other named unary operator. The operator may be any of:

    -r  File is readable by effective uid/gid.
    -w  File is writable by effective uid/gid.
    -x  File is executable by effective uid/gid.
    -o  File is owned by effective uid.

    -R  File is readable by real uid/gid.
    -W  File is writable by real uid/gid.
    -X  File is executable by real uid/gid.
    -O  File is owned by real uid.

    -e  File exists.
    -z  File has zero size (is empty).
    -s  File has nonzero size (returns size in bytes).

    -f  File is a plain file.
    -d  File is a directory.
    -l  File is a symbolic link (false if symlinks aren't
        supported by the file system).
    -p  File is a named pipe (FIFO), or Filehandle is a pipe.
    -S  File is a socket.
    -b  File is a block special file.
    -c  File is a character special file.
    -t  Filehandle is opened to a tty.

    -u  File has setuid bit set.
    -g  File has setgid bit set.
    -k  File has sticky bit set.

    -T  File is an ASCII or UTF-8 text file (heuristic guess).
    -B  File is a "binary" file (opposite of -T).

    -M  Script start time minus file modification time, in days.
    -A  Same for access time.
    -C  Same for inode change time (Unix, may differ for other
	platforms)

Example:

while (<>) {
    chomp;
    next unless -f $_;  # ignore specials
    #...
}

Note that -s/a/b/ does not do a negated substitution. Saying -exp($foo) still works as expected, however: only single letters following a minus are interpreted as file tests.

These operators are exempt from the "looks like a function rule" described above. That is, an opening parenthesis after the operator does not affect how much of the following code constitutes the argument. Put the opening parentheses before the operator to separate it from code that follows (this applies only to operators with higher precedence than unary operators, of course):

-s($file) + 1024   # probably wrong; same as -s($file + 1024)
(-s $file) + 1024  # correct

The interpretation of the file permission operators -r, -R, -w, -W, -x, and -X is by default based solely on the mode of the file and the uids and gids of the user. There may be other reasons you can't actually read, write, or execute the file: for example network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats. Note that the use of these six specific operators to verify if some operation is possible is usually a mistake, because it may be open to race conditions.

Also note that, for the superuser on the local filesystems, the -r, -R, -w, and -W tests always return 1, and -x and -X return 1 if any execute bit is set in the mode. Scripts run by the superuser may thus need to do a stat to determine the actual mode of the file, or temporarily set their effective uid to something else.

If you are using ACLs, there is a pragma called filetest that may produce more accurate results than the bare stat mode bits. When under use filetest 'access', the above-mentioned filetests test whether the permission can(not) be granted using the access(2) family of system calls. Also note that the -x and -X tests may under this pragma return true even if there are no execute permission bits set (nor any extra execute permission ACLs). This strangeness is due to the underlying system calls' definitions. Note also that, due to the implementation of use filetest 'access', the _ special filehandle won't cache the results of the file tests when this pragma is in effect. Read the documentation for the filetest pragma for more information.

The -T and -B tests work as follows. The first block or so of the file is examined to see if it is valid UTF-8 that includes non-ASCII characters. If so, it's a -T file. Otherwise, that same portion of the file is examined for odd characters such as strange control codes or characters with the high bit set. If more than a third of the characters are strange, it's a -B file; otherwise it's a -T file. Also, any file containing a zero byte in the examined portion is considered a binary file. (If executed within the scope of a use locale which includes LC_CTYPE, odd characters are anything that isn't a printable nor space in the current locale.) If -T or -B is used on a filehandle, the current IO buffer is examined rather than the first block. Both -T and -B return true on an empty file, or a file at EOF when testing a filehandle. Because you have to read a file to do the -T test, on most occasions you want to use a -f against the file first, as in next unless -f $file && -T $file.

If any of the file tests (or either the stat or lstat operator) is given the special filehandle consisting of a solitary underline, then the stat structure of the previous file test (or stat operator) is used, saving a system call. (This doesn't work with -t, and you need to remember that lstat and -l leave values in the stat structure for the symbolic link, not the real file.) (Also, if the stat buffer was filled by an lstat call, -T and -B will reset it with the results of stat _). Example:

print "Can do.\n" if -r $a || -w _ || -x _;

stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;

As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file test operators, in a way that -f -w -x $file is equivalent to -x $file && -w _ && -f _. (This is only fancy syntax: if you use the return value of -f $file as an argument to another filetest operator, no special magic will happen.)

Portability issues: "-X" in perlport.

To avoid confusing would-be users of your code with mysterious syntax errors, put something like this at the top of your script:

use 5.010;  # so filetest ops can stack

abs VALUE

abs

Returns the absolute value of its argument. If VALUE is omitted, uses $_.

accept NEWSOCKET,GENERICSOCKET

Accepts an incoming socket connect, just as accept(2) does. Returns the packed address if it succeeded, false otherwise. See the example in "Sockets: Client/Server Communication" in perlipc.

On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See "$^F" in perlvar.

alarm SECONDS

alarm

Arranges to have a SIGALRM delivered to this process after the specified number of wallclock seconds has elapsed. If SECONDS is not specified, the value stored in $_ is used. (On some machines, unfortunately, the elapsed time may be up to one second less or more than you specified because of how seconds are counted, and process scheduling may delay the delivery of the signal even further.)

Only one timer may be counting at once. Each call disables the previous timer, and an argument of 0 may be supplied to cancel the previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer.

For delays of finer granularity than one second, the Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) provides ualarm. You may also use Perl's four-argument version of select leaving the first three arguments undefined, or you might be able to use the syscall interface to access setitimer(2) if your system supports it. See perlfaq8 for details.

It is usually a mistake to intermix alarm and sleep calls, because sleep may be internally implemented on your system with alarm.

If you want to use alarm to time out a system call you need to use an eval/die pair. You can't rely on the alarm causing the system call to fail with $! set to EINTR because Perl sets up signal handlers to restart system calls on some systems. Using eval/die always works, modulo the caveats given in "Signals" in perlipc.

eval {
    local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
    alarm $timeout;
    my $nread = sysread $socket, $buffer, $size;
    alarm 0;
};
if ($@) {
    die unless $@ eq "alarm\n";   # propagate unexpected errors
    # timed out
}
else {
    # didn't
}

For more information see perlipc.

Portability issues: "alarm" in perlport.

atan2 Y,X

Returns the arctangent of Y/X in the range -PI to PI.

For the tangent operation, you may use the Math::Trig::tan function, or use the familiar relation:

sub tan { sin($_[0]) / cos($_[0])  }

The return value for atan2(0,0) is implementation-defined; consult your atan2(3) manpage for more information.

Portability issues: "atan2" in perlport.

bind SOCKET,NAME

Binds a network address to a socket, just as bind(2) does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the appropriate type for the socket. See the examples in "Sockets: Client/Server Communication" in perlipc.

binmode FILEHANDLE, LAYER

binmode FILEHANDLE

Arranges for FILEHANDLE to be read or written in "binary" or "text" mode on systems where the run-time libraries distinguish between binary and text files. If FILEHANDLE is an expression, the value is taken as the name of the filehandle. Returns true on success, otherwise it returns undef and sets $! (errno).

On some systems (in general, DOS- and Windows-based systems) binmode is necessary when you're not working with a text file. For the sake of portability it is a good idea always to use it when appropriate, and never to use it when it isn't appropriate. Also, people can set their I/O to be by default UTF8-encoded Unicode, not bytes.

In other words: regardless of platform, use binmode on binary data, like images, for example.

If LAYER is present it is a single string, but may contain multiple directives. The directives alter the behaviour of the filehandle. When LAYER is present, using binmode on a text file makes sense.

If LAYER is omitted or specified as :raw the filehandle is made suitable for passing binary data. This includes turning off possible CRLF translation and marking it as bytes (as opposed to Unicode characters). Note that, despite what may be implied in "Programming Perl" (the Camel, 3rd edition) or elsewhere, :raw is not simply the inverse of :crlf. Other layers that would affect the binary nature of the stream are also disabled. See PerlIO, perlrun, and the discussion about the PERLIO environment variable.

The :bytes, :crlf, :utf8, and any other directives of the form :..., are called I/O layers. The open pragma can be used to establish default I/O layers.

The LAYER parameter of the binmode function is described as "DISCIPLINE" in "Programming Perl, 3rd Edition". However, since the publishing of this book, by many known as "Camel III", the consensus of the naming of this functionality has moved from "discipline" to "layer". All documentation of this version of Perl therefore refers to "layers" rather than to "disciplines". Now back to the regularly scheduled documentation...

To mark FILEHANDLE as UTF-8, use :utf8 or :encoding(UTF-8). :utf8 just marks the data as UTF-8 without further checking, while :encoding(UTF-8) checks the data for actually being valid UTF-8. More details can be found in PerlIO::encoding.

In general, binmode should be called after open but before any I/O is done on the filehandle. Calling binmode normally flushes any pending buffered output data (and perhaps pending input data) on the handle. An exception to this is the :encoding layer that changes the default character encoding of the handle. The :encoding layer sometimes needs to be called in mid-stream, and it doesn't flush the stream. :encoding also implicitly pushes on top of itself the :utf8 layer because internally Perl operates on UTF8-encoded Unicode characters.

The operating system, device drivers, C libraries, and Perl run-time system all conspire to let the programmer treat a single character (\n) as the line terminator, irrespective of external representation. On many operating systems, the native text file representation matches the internal representation, but on some platforms the external representation of \n is made up of more than one character.

All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use a single character to end each line in the external representation of text (even though that single character is CARRIAGE RETURN on old, pre-Darwin flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other systems like OS/2, DOS, and the various flavors of MS-Windows, your program sees a \n as a simple \cJ, but what's stored in text files are the two characters \cM\cJ. That means that if you don't use binmode on these systems, \cM\cJ sequences on disk will be converted to \n on input, and any \n in your program will be converted back to \cM\cJ on output. This is what you want for text files, but it can be disastrous for binary files.

Another consequence of using binmode (on some systems) is that special end-of-file markers will be seen as part of the data stream. For systems from the Microsoft family this means that, if your binary data contain \cZ, the I/O subsystem will regard it as the end of the file, unless you use binmode.

binmode is important not only for readline and print operations, but also when using read, seek, sysread, syswrite and tell (see perlport for more details). See the $/ and $\ variables in perlvar for how to manually set your input and output line-termination sequences.

Portability issues: "binmode" in perlport.

bless REF,CLASSNAME

bless REF

This function tells the thingy referenced by REF that it is now an object in the CLASSNAME package. If CLASSNAME is omitted, the current package is used. Because a bless is often the last thing in a constructor, it returns the reference for convenience. Always use the two-argument version if a derived class might inherit the method doing the blessing. See perlobj for more about the blessing (and blessings) of objects.

Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all lowercase names are considered reserved for Perl pragmas. Builtin types have all uppercase names. To prevent confusion, you may wish to avoid such package names as well. Make sure that CLASSNAME is a true value.

See "Perl Modules" in perlmod.

break

Break out of a given block.

break is available only if the "switch" feature is enabled or if it is prefixed with CORE::. The "switch" feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope.

caller EXPR

caller

Returns the context of the current pure perl subroutine call. In scalar context, returns the caller's package name if there is a caller (that is, if we're in a subroutine or eval or require) and the undefined value otherwise. caller never returns XS subs and they are skipped. The next pure perl sub will appear instead of the XS sub in caller's return values. In list context, caller returns

   # 0         1          2
my ($package, $filename, $line) = caller;

With EXPR, it returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.

   #  0         1          2      3            4
my ($package, $filename, $line, $subroutine, $hasargs,

   #  5          6          7            8       9         10
   $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
 = caller($i);

Here, $subroutine is the function that the caller called (rather than the function containing the caller). Note that $subroutine may be (eval) if the frame is not a subroutine call, but an eval. In such a case additional elements $evaltext and $is_require are set: $is_require is true if the frame is created by a require or use statement, $evaltext contains the text of the eval EXPR statement. In particular, for an eval BLOCK statement, $subroutine is (eval), but $evaltext is undefined. (Note also that each use statement creates a require frame inside an eval EXPR frame.) $subroutine may also be (unknown) if this particular subroutine happens to have been deleted from the symbol table. $hasargs is true if a new instance of @_ was set up for the frame. $hints and $bitmask contain pragmatic hints that the caller was compiled with. $hints corresponds to $^H, and $bitmask corresponds to ${^WARNING_BITS}. The $hints and $bitmask values are subject to change between versions of Perl, and are not meant for external use.

$hinthash is a reference to a hash containing the value of %^H when the caller was compiled, or undef if %^H was empty. Do not modify the values of this hash, as they are the actual values stored in the optree.

Furthermore, when called from within the DB package in list context, and with an argument, caller returns more detailed information: it sets the list variable @DB::args to be the arguments with which the subroutine was invoked.

Be aware that the optimizer might have optimized call frames away before caller had a chance to get the information. That means that caller(N) might not return information about the call frame you expect it to, for N > 1. In particular, @DB::args might have information from the previous time caller was called.

Be aware that setting @DB::args is best effort, intended for debugging or generating backtraces, and should not be relied upon. In particular, as @_ contains aliases to the caller's arguments, Perl does not take a copy of @_, so @DB::args will contain modifications the subroutine makes to @_ or its contents, not the original values at call time. @DB::args, like @_, does not hold explicit references to its elements, so under certain cases its elements may have become freed and reallocated for other variables or temporary values. Finally, a side effect of the current implementation is that the effects of shift @_ can normally be undone (but not pop @_ or other splicing, and not if a reference to @_ has been taken, and subject to the caveat about reallocated elements), so @DB::args is actually a hybrid of the current state and initial state of @_. Buyer beware.

chdir EXPR

chdir FILEHANDLE

chdir DIRHANDLE

chdir

Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to the directory specified by $ENV{HOME}, if set; if not, changes to the directory specified by $ENV{LOGDIR}. (Under VMS, the variable $ENV{'SYS$LOGIN'} is also checked, and used if it is set.) If neither is set, chdir does nothing and fails. It returns true on success, false otherwise. See the example under die.

On systems that support fchdir(2), you may pass a filehandle or directory handle as the argument. On systems that don't support fchdir(2), passing handles raises an exception.

chmod LIST

Changes the permissions of a list of files. The first element of the list must be the numeric mode, which should probably be an octal number, and which definitely should not be a string of octal digits: 0644 is okay, but "0644" is not. Returns the number of files successfully changed. See also oct if all you have is a string.

my $cnt = chmod 0755, "foo", "bar";
chmod 0755, @executables;
my $mode = "0644"; chmod $mode, "foo";      # !!! sets mode to
                                            # --w----r-T
my $mode = "0644"; chmod oct($mode), "foo"; # this is better
my $mode = 0644;   chmod $mode, "foo";      # this is best

On systems that support fchmod(2), you may pass filehandles among the files. On systems that don't support fchmod(2), passing filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized; barewords are considered filenames.

open(my $fh, "<", "foo");
my $perm = (stat $fh)[2] & 07777;
chmod($perm | 0600, $fh);

You can also import the symbolic S_I* constants from the Fcntl module:

use Fcntl qw( :mode );
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# Identical to the chmod 0755 of the example above.

Portability issues: "chmod" in perlport.

chomp VARIABLE

chomp( LIST )

chomp

This safer version of chop removes any trailing string that corresponds to the current value of $/ (also known as $INPUT_RECORD_SEPARATOR in the English module). It returns the total number of characters removed from all its arguments. It's often used to remove the newline from the end of an input record when you're worried that the final record may be missing its newline. When in paragraph mode ($/ = ''), it removes all trailing newlines from the string. When in slurp mode ($/ = undef) or fixed-length record mode ($/ is a reference to an integer or the like; see perlvar), chomp won't remove anything. If VARIABLE is omitted, it chomps $_. Example:

while (<>) {
    chomp;  # avoid \n on last field
    my @array = split(/:/);
    # ...
}

If VARIABLE is a hash, it chomps the hash's values, but not its keys, resetting the each iterator in the process.

You can actually chomp anything that's an lvalue, including an assignment:

chomp(my $cwd = `pwd`);
chomp(my $answer = <STDIN>);

If you chomp a list, each element is chomped, and the total number of characters removed is returned.

Note that parentheses are necessary when you're chomping anything that is not a simple variable. This is because chomp $cwd = `pwd`; is interpreted as (chomp $cwd) = `pwd`;, rather than as chomp( $cwd = `pwd` ) which you might expect. Similarly, chomp $a, $b is interpreted as chomp($a), $b rather than as chomp($a, $b).

chop VARIABLE

chop( LIST )

chop

Chops off the last character of a string and returns the character chopped. It is much more efficient than s/.$//s because it neither scans nor copies the string. If VARIABLE is omitted, chops $_. If VARIABLE is a hash, it chops the hash's values, but not its keys, resetting the each iterator in the process.

You can actually chop anything that's an lvalue, including an assignment.

If you chop a list, each element is chopped. Only the value of the last chop is returned.

Note that chop returns the last character. To return all but the last character, use substr($string, 0, -1).

See also each, values, and sort.

kill SIGNAL, LIST

kill SIGNAL

Sends a signal to a list of processes. Returns the number of arguments that were successfully used to signal (which is not necessarily the same as the number of processes actually killed, e.g. where a process group is killed).

my $cnt = kill 'HUP', $child1, $child2;
kill 'KILL', @goners;

SIGNAL may be either a signal name (a string) or a signal number. A signal name may start with a SIG prefix, thus FOO and SIGFOO refer to the same signal. The string form of SIGNAL is recommended for portability because the same signal may have different numbers in different operating systems.

A list of signal names supported by the current platform can be found in $Config{sig_name}, which is provided by the Config module. See Config for more details.

A negative signal name is the same as a negative signal number, killing process groups instead of processes. For example, kill '-KILL', $pgrp and kill -9, $pgrp will send SIGKILL to the entire process group specified. That means you usually want to use positive not negative signals.

If SIGNAL is either the number 0 or the string ZERO (or SIGZERO), no signal is sent to the process, but kill checks whether it's possible to send a signal to it (that means, to be brief, that the process is owned by the same user, or we are the super-user). This is useful to check that a child process is still alive (even if only as a zombie) and hasn't changed its UID. See perlport for notes on the portability of this construct.

The behavior of kill when a PROCESS number is zero or negative depends on the operating system. For example, on POSIX-conforming systems, zero will signal the current process group, -1 will signal all processes, and any other negative PROCESS number will act as a negative signal number and kill the entire process group specified.

If both the SIGNAL and the PROCESS are negative, the results are undefined. A warning may be produced in a future version.

See "Signals" in perlipc for more details.

On some platforms such as Windows where the fork(2) system call is not available, Perl can be built to emulate fork at the interpreter level. This emulation has limitations related to kill that have to be considered, for code running on Windows and in code intended to be portable.

See perlfork for more details.

If there is no LIST of processes, no signal is sent, and the return value is 0. This form is sometimes used, however, because it causes tainting checks to be run. But see "Laundering and Detecting Tainted Data" in perlsec.

Portability issues: "kill" in perlport.

last LABEL

last EXPR

last

The last command is like the break statement in C (as used in loops); it immediately exits the loop in question. If the LABEL is omitted, the command refers to the innermost enclosing loop. The last EXPR form, available starting in Perl 5.18.0, allows a label name to be computed at run time, and is otherwise identical to last LABEL. The continue block, if any, is not executed:

LINE: while (<STDIN>) {
    last LINE if /^$/;  # exit when done with header
    #...
}

last cannot be used to exit a block that returns a value such as eval {}, sub {}, or do {}, and should not be used to exit a grep or map operation.

Note that a block by itself is semantically identical to a loop that executes once. Thus last can be used to effect an early exit out of such a block.

See also continue for an illustration of how last, next, and redo work.

Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so last ("foo")."bar" will cause "bar" to be part of the argument to last.

lc EXPR

lc

Returns a lowercased version of EXPR. This is the internal function implementing the \L escape in double-quoted strings.

If EXPR is omitted, uses $_.

What gets returned depends on several factors:

If use bytes is in effect:

The results follow ASCII rules. Only the characters A-Z change, to a-z respectively.

Otherwise, if use locale for LC_CTYPE is in effect:

Respects current LC_CTYPE locale for code points < 256; and uses Unicode rules for the remaining code points (this last can only happen if the UTF8 flag is also set). See perllocale.

Starting in v5.20, Perl uses full Unicode rules if the locale is UTF-8. Otherwise, there is a deficiency in this scheme, which is that case changes that cross the 255/256 boundary are not well-defined. For example, the lower case of LATIN CAPITAL LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII platforms). But under use locale (prior to v5.20 or not a UTF-8 locale), the lower case of U+1E9E is itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the current locale, and Perl has no way of knowing if that character even exists in the locale, much less what code point it is. Perl returns a result that is above 255 (almost always the input character unchanged), for all instances (and there aren't many) where the 255/256 boundary would otherwise be crossed; and starting in v5.22, it raises a locale warning.

Otherwise, If EXPR has the UTF8 flag set:

Unicode rules are used for the case change.

Otherwise, if use feature 'unicode_strings' or use locale ':not_characters' is in effect:

Unicode rules are used for the case change.

Otherwise:

ASCII rules are used for the case change. The lowercase of any character outside the ASCII range is the character itself.

lcfirst EXPR

lcfirst

Returns the value of EXPR with the first character lowercased. This is the internal function implementing the \l escape in double-quoted strings.

If EXPR is omitted, uses $_.

This function behaves the same way under various pragmas, such as in a locale, as lc does.

length EXPR

length

Returns the length in characters of the value of EXPR. If EXPR is omitted, returns the length of $_. If EXPR is undefined, returns undef.

This function cannot be used on an entire array or hash to find out how many elements these have. For that, use scalar @array and scalar keys %hash, respectively.

Like all Perl character operations, length normally deals in logical characters, not physical bytes. For how many bytes a string encoded as UTF-8 would take up, use length(Encode::encode_utf8(EXPR)) (you'll have to use Encode first). See Encode and perlunicode.

__LINE__

A special token that compiles to the current line number.

link OLDFILE,NEWFILE

Creates a new filename linked to the old filename. Returns true for success, false otherwise.

Portability issues: "link" in perlport.

listen SOCKET,QUEUESIZE

Does the same thing that the listen(2) system call does. Returns true if it succeeded, false otherwise. See the example in "Sockets: Client/Server Communication" in perlipc.

local EXPR

You really probably want to be using my instead, because local isn't what most people think of as "local". See "Private Variables via my()" in perlsub for details.

A local modifies the listed variables to be local to the enclosing block, file, or eval. If more than one value is listed, the list must be placed in parentheses. See "Temporary Values via local()" in perlsub for details, including issues with tied arrays and hashes.

The delete local EXPR construct can also be used to localize the deletion of array/hash elements to the current block. See "Localized deletion of elements of composite types" in perlsub.

localtime EXPR

localtime

Converts a time as returned by the time function to a 9-element list with the time analyzed for the local time zone. Typically used as follows:

#     0    1    2     3     4    5     6     7     8
my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                            localtime(time);

All list elements are numeric and come straight out of the C `struct tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the specified time.

$mday is the day of the month and $mon the month in the range 0..11, with 0 indicating January and 11 indicating December. This makes it easy to get a month name from a list:

my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
print "$abbr[$mon] $mday";
# $mon=9, $mday=18 gives "Oct 18"

$year contains the number of years since 1900. To get a 4-digit year write:

$year += 1900;

To get the last two digits of the year (e.g., "01" in 2001) do:

$year = sprintf("%02d", $year % 100);

$wday is the day of the week, with 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of the year, in the range 0..364 (or 0..365 in leap years.)

$isdst is true if the specified time occurs during Daylight Saving Time, false otherwise.

If EXPR is omitted, localtime uses the current time (as returned by time).

In scalar context, localtime returns the ctime(3) value:

my $now_string = localtime;  # e.g., "Thu Oct 13 04:54:34 1994"

The format of this scalar value is not locale-dependent but built into Perl. For GMT instead of local time use the gmtime builtin. See also the Time::Local module (for converting seconds, minutes, hours, and such back to the integer value returned by time), and the POSIX module's strftime and mktime functions.

To get somewhat similar but locale-dependent date strings, set up your locale environment variables appropriately (please see perllocale) and try for example:

use POSIX qw(strftime);
my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
# or for GMT formatted appropriately for your locale:
my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;

Note that %a and %b, the short forms of the day of the week and the month of the year, may not necessarily be three characters wide.

The Time::gmtime and Time::localtime modules provide a convenient, by-name access mechanism to the gmtime and localtime functions, respectively.

For a comprehensive date and time representation look at the DateTime module on CPAN.

Portability issues: "localtime" in perlport.

lock THING

This function places an advisory lock on a shared variable or referenced object contained in THING until the lock goes out of scope.

The value returned is the scalar itself, if the argument is a scalar, or a reference, if the argument is a hash, array or subroutine.

lock is a "weak keyword"; this means that if you've defined a function by this name (before any calls to it), that function will be called instead. If you are not under use threads::shared this does nothing. See threads::shared.

log EXPR

log

Returns the natural logarithm (base e) of EXPR. If EXPR is omitted, returns the log of $_. To get the log of another base, use basic algebra: The base-N log of a number is equal to the natural log of that number divided by the natural log of N. For example:

sub log10 {
    my $n = shift;
    return log($n)/log(10);
}

See also exp for the inverse operation.

lstat FILEHANDLE

lstat EXPR

lstat DIRHANDLE

lstat

Does the same thing as the stat function (including setting the special _ filehandle) but stats a symbolic link instead of the file the symbolic link points to. If symbolic links are unimplemented on your system, a normal stat is done. For much more detailed information, please see the documentation for stat.

If EXPR is omitted, stats $_.

Portability issues: "lstat" in perlport.

m//

The match operator. See "Regexp Quote-Like Operators" in perlop.

map BLOCK LIST

map EXPR,LIST

Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and returns the list value composed of the results of each such evaluation. In scalar context, returns the total number of elements so generated. Evaluates BLOCK or EXPR in list context, so each element of LIST may produce zero, one, or more elements in the returned value.

my @chars = map(chr, @numbers);

translates a list of numbers to the corresponding characters.

my @squares = map { $_ * $_ } @numbers;

translates a list of numbers to their squared values.

my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;

shows that number of returned elements can differ from the number of input elements. To omit an element, return an empty list (). This could also be achieved by writing

my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;

which makes the intention more clear.

Map always returns a list, which can be assigned to a hash such that the elements become key/value pairs. See perldata for more details.

my %hash = map { get_a_key_for($_) => $_ } @array;

is just a funny way to write

my %hash;
foreach (@array) {
    $hash{get_a_key_for($_)} = $_;
}

Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST. While this is useful and supported, it can cause bizarre results if the elements of LIST are not variables. Using a regular foreach loop for this purpose would be clearer in most cases. See also grep for an array composed of those items of the original list for which the BLOCK or EXPR evaluates to true.

{ starts both hash references and blocks, so map { ... could be either the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look ahead for the closing } it has to take a guess at which it's dealing with based on what it finds just after the {. Usually it gets it right, but if it doesn't it won't realize something is wrong until it gets to the } and encounters the missing (or unexpected) comma. The syntax error will be reported close to the }, but you'll need to change something near the { such as using a unary + or semicolon to give Perl some help:

my %hash = map {  "\L$_" => 1  } @array # perl guesses EXPR. wrong
my %hash = map { +"\L$_" => 1  } @array # perl guesses BLOCK. right
my %hash = map {; "\L$_" => 1  } @array # this also works
my %hash = map { ("\L$_" => 1) } @array # as does this
my %hash = map {  lc($_) => 1  } @array # and this.
my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!

my %hash = map  ( lc($_), 1 ),   @array # evaluates to (1, @array)

or to force an anon hash constructor use +{:

my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
                                          # comma at end

to get a list of anonymous hashes each with only one entry apiece.

mkdir FILENAME,MASK

mkdir FILENAME

mkdir

Creates the directory specified by FILENAME, with permissions specified by MASK (as modified by umask). If it succeeds it returns true; otherwise it returns false and sets $! (errno). MASK defaults to 0777 if omitted, and FILENAME defaults to $_ if omitted.

In general, it is better to create directories with a permissive MASK and let the user modify that with their umask than it is to supply a restrictive MASK and give the user no way to be more permissive. The exceptions to this rule are when the file or directory should be kept private (mail files, for instance). The documentation for umask discusses the choice of MASK in more detail.

Note that according to the POSIX 1003.1-1996 the FILENAME may have any number of trailing slashes. Some operating and filesystems do not get this right, so Perl automatically removes all trailing slashes to keep everyone happy.

To recursively create a directory structure, look at the make_path function of the File::Path module.

msgctl ID,CMD,ARG

Calls the System V IPC function msgctl(2). You'll probably have to say

use IPC::SysV;

first to get the correct constant definitions. If CMD is IPC_STAT, then ARG must be a variable that will hold the returned msqid_ds structure. Returns like ioctl: the undefined value for error, "0 but true" for zero, or the actual return value otherwise. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Semaphore.

Portability issues: "msgctl" in perlport.

msgget KEY,FLAGS

Calls the System V IPC function msgget(2). Returns the message queue id, or undef on error. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Msg.

Portability issues: "msgget" in perlport.

msgrcv ID,VAR,SIZE,TYPE,FLAGS

Calls the System V IPC function msgrcv to receive a message from message queue ID into variable VAR with a maximum message size of SIZE. Note that when a message is received, the message type as a native long integer will be the first thing in VAR, followed by the actual message. This packing may be opened with unpack("l! a*"). Taints the variable. Returns true if successful, false on error. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Msg.

Portability issues: "msgrcv" in perlport.

msgsnd ID,MSG,FLAGS

Calls the System V IPC function msgsnd to send the message MSG to the message queue ID. MSG must begin with the native long integer message type, be followed by the length of the actual message, and then finally the message itself. This kind of packing can be achieved with pack("l! a*", $type, $message). Returns true if successful, false on error. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Msg.

Portability issues: "msgsnd" in perlport.

my VARLIST

my TYPE VARLIST

my VARLIST : ATTRS

my TYPE VARLIST : ATTRS

A my declares the listed variables to be local (lexically) to the enclosing block, file, or eval. If more than one variable is listed, the list must be placed in parentheses.

The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE may be a bareword, a constant declared with use constant, or __PACKAGE__. It is currently bound to the use of the fields pragma, and attributes are handled using the attributes pragma, or starting from Perl 5.8.0 also via the Attribute::Handlers module. See "Private Variables via my()" in perlsub for details.

Note that with a parenthesised list, undef can be used as a dummy placeholder, for example to skip assignment of initial values:

my ( undef, $min, $hour ) = localtime;

next LABEL

next EXPR

next

The next command is like the continue statement in C; it starts the next iteration of the loop:

LINE: while (<STDIN>) {
    next LINE if /^#/;  # discard comments
    #...
}

Note that if there were a continue block on the above, it would get executed even on discarded lines. If LABEL is omitted, the command refers to the innermost enclosing loop. The next EXPR form, available as of Perl 5.18.0, allows a label name to be computed at run time, being otherwise identical to next LABEL.

next cannot be used to exit a block which returns a value such as eval {}, sub {}, or do {}, and should not be used to exit a grep or map operation.

Note that a block by itself is semantically identical to a loop that executes once. Thus next will exit such a block early.

See also continue for an illustration of how last, next, and redo work.

Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so next ("foo")."bar" will cause "bar" to be part of the argument to next.

no MODULE VERSION LIST

no MODULE VERSION

no MODULE LIST

no MODULE

no VERSION

See the use function, of which no is the opposite.

oct EXPR

oct

Interprets EXPR as an octal string and returns the corresponding value. (If EXPR happens to start off with 0x, interprets it as a hex string. If EXPR starts off with 0b, it is interpreted as a binary string. Leading whitespace is ignored in all three cases.) The following will handle decimal, binary, octal, and hex in standard Perl notation:

$val = oct($val) if $val =~ /^0/;

If EXPR is omitted, uses $_. To go the other way (produce a number in octal), use sprintf or printf:

my $dec_perms = (stat("filename"))[2] & 07777;
my $oct_perm_str = sprintf "%o", $perms;

The oct function is commonly used when a string such as 644 needs to be converted into a file mode, for example. Although Perl automatically converts strings into numbers as needed, this automatic conversion assumes base 10.

Leading white space is ignored without warning, as too are any trailing non-digits, such as a decimal point (oct only handles non-negative integers, not negative integers or floating point).

open FILEHANDLE,EXPR

open FILEHANDLE,MODE,EXPR

open FILEHANDLE,MODE,EXPR,LIST

open FILEHANDLE,MODE,REFERENCE

open FILEHANDLE

Opens the file whose filename is given by EXPR, and associates it with FILEHANDLE.

Simple examples to open a file for reading:

    open(my $fh, "<", "input.txt")
	or die "Can't open < input.txt: $!";

and for writing:

    open(my $fh, ">", "output.txt")
	or die "Can't open > output.txt: $!";

(The following is a comprehensive reference to open: for a gentler introduction you may consider perlopentut.)

If FILEHANDLE is an undefined scalar variable (or array or hash element), a new filehandle is autovivified, meaning that the variable is assigned a reference to a newly allocated anonymous filehandle. Otherwise if FILEHANDLE is an expression, its value is the real filehandle. (This is considered a symbolic reference, so use strict "refs" should not be in effect.)

If three (or more) arguments are specified, the open mode (including optional encoding) in the second argument are distinct from the filename in the third. If MODE is < or nothing, the file is opened for input. If MODE is >, the file is opened for output, with existing files first being truncated ("clobbered") and nonexisting files newly created. If MODE is >>, the file is opened for appending, again being created if necessary.

You can put a + in front of the > or < to indicate that you want both read and write access to the file; thus +< is almost always preferred for read/write updates--the +> mode would clobber the file first. You can't usually use either read-write mode for updating textfiles, since they have variable-length records. See the -i switch in perlrun for a better approach. The file is created with permissions of 0666 modified by the process's umask value.

These various prefixes correspond to the fopen(3) modes of r, r+, w, w+, a, and a+.

In the one- and two-argument forms of the call, the mode and filename should be concatenated (in that order), preferably separated by white space. You can--but shouldn't--omit the mode in these forms when that mode is <. It is safe to use the two-argument form of open if the filename argument is a known literal.

For three or more arguments if MODE is |-, the filename is interpreted as a command to which output is to be piped, and if MODE is -|, the filename is interpreted as a command that pipes output to us. In the two-argument (and one-argument) form, one should replace dash (-) with the command. See "Using open() for IPC" in perlipc for more examples of this. (You are not allowed to open to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and "Bidirectional Communication with Another Process" in perlipc for alternatives.)

In the form of pipe opens taking three or more arguments, if LIST is specified (extra arguments after the command name) then LIST becomes arguments to the command invoked if the platform supports it. The meaning of open with more than three arguments for non-pipe modes is not yet defined, but experimental "layers" may give extra LIST arguments meaning.

In the two-argument (and one-argument) form, opening <- or - opens STDIN and opening >- opens STDOUT.

You may (and usually should) use the three-argument form of open to specify I/O layers (sometimes referred to as "disciplines") to apply to the handle that affect how the input and output are processed (see open and PerlIO for more details). For example:

open(my $fh, "<:encoding(UTF-8)", $filename)
  || die "Can't open UTF-8 encoded $filename: $!";

opens the UTF8-encoded file containing Unicode characters; see perluniintro. Note that if layers are specified in the three-argument form, then default layers stored in ${^OPEN} (see perlvar; usually set by the open pragma or the switch -CioD) are ignored. Those layers will also be ignored if you specifying a colon with no name following it. In that case the default layer for the operating system (:raw on Unix, :crlf on Windows) is used.

Open returns nonzero on success, the undefined value otherwise. If the open involved a pipe, the return value happens to be the pid of the subprocess.

When opening a file, it's seldom a good idea to continue if the request failed, so open is frequently used with die. Even if die won't do what you want (say, in a CGI script, where you want to format a suitable error message (but there are modules that can help with that problem)) always check the return value from opening a file.

The filehandle will be closed when its reference count reaches zero. If it is a lexically scoped variable declared with my, that usually means the end of the enclosing scope. However, this automatic close does not check for errors, so it is better to explicitly close filehandles, especially those used for writing:

close($handle)
   || warn "close failed: $!";

An older style is to use a bareword as the filehandle, as

open(FH, "<", "input.txt")
   or die "Can't open < input.txt: $!";

Then you can use FH as the filehandle, in close FH and <FH> and so on. Note that it's a global variable, so this form is not recommended in new code.

As a shortcut a one-argument call takes the filename from the global scalar variable of the same name as the filehandle:

$ARTICLE = 100;
open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";

Here $ARTICLE must be a global (package) scalar variable - not one declared with my or state.

As a special case the three-argument form with a read/write mode and the third argument being undef:

open(my $tmp, "+>", undef) or die ...

opens a filehandle to an anonymous temporary file. Also using +< works for symmetry, but you really should consider writing something to the temporary file first. You will need to seek to do the reading.

Perl is built using PerlIO by default. Unless you've changed this (such as building Perl with Configure -Uuseperlio), you can open filehandles directly to Perl scalars via:

open(my $fh, ">", \$variable) || ..

To (re)open STDOUT or STDERR as an in-memory file, close it first:

    close STDOUT;
    open(STDOUT, ">", \$variable)
	or die "Can't open STDOUT: $!";

See perliol for detailed info on PerlIO.

General examples:

open(my $log, ">>", "/usr/spool/news/twitlog");
# if the open fails, output is discarded

open(my $dbase, "+<", "dbase.mine")      # open for update
    or die "Can't open 'dbase.mine' for update: $!";

open(my $dbase, "+<dbase.mine")          # ditto
    or die "Can't open 'dbase.mine' for update: $!";

open(my $article_fh, "-|", "caesar <$article")  # decrypt
                                                # article
    or die "Can't start caesar: $!";

open(my $article_fh, "caesar <$article |")      # ditto
    or die "Can't start caesar: $!";

open(my $out_fh, "|-", "sort >Tmp$$")    # $$ is our process id
    or die "Can't start sort: $!";

# in-memory files
open(my $memory, ">", \$var)
    or die "Can't open memory file: $!";
print $memory "foo!\n";              # output will appear in $var

You may also, in the Bourne shell tradition, specify an EXPR beginning with >&, in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if numeric) to be duped (as in dup(2)) and opened. You may use & after >, >>, <, +>, +>>, and +<. The mode you specify should match the mode of the original filehandle. (Duping a filehandle does not take into account any existing contents of IO buffers.) If you use the three-argument form, then you can pass either a number, the name of a filehandle, or the normal "reference to a glob".

Here is a script that saves, redirects, and restores STDOUT and STDERR using various methods:

#!/usr/bin/perl
open(my $oldout, ">&STDOUT")     or die "Can't dup STDOUT: $!";
open(OLDERR,     ">&", \*STDERR) or die "Can't dup STDERR: $!";

open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
open(STDERR, ">&STDOUT")     or die "Can't dup STDOUT: $!";

select STDERR; $| = 1;  # make unbuffered
select STDOUT; $| = 1;  # make unbuffered

print STDOUT "stdout 1\n";  # this works for
print STDERR "stderr 1\n";  # subprocesses too

open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
open(STDERR, ">&OLDERR")    or die "Can't dup OLDERR: $!";

print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";

If you specify '<&=X', where X is a file descriptor number or a filehandle, then Perl will do an equivalent of C's fdopen(3) of that file descriptor (and not call dup(2)); this is more parsimonious of file descriptors. For example:

# open for input, reusing the fileno of $fd
open(my $fh, "<&=", $fd)

open(my $fh, "<&=$fd")

# open for append, using the fileno of $oldfh
open(my $fh, ">>&=", $oldfh)

Being parsimonious on filehandles is also useful (besides being parsimonious) for example when something is dependent on file descriptors, like for example locking using flock. If you do just open(my $A, ">>&", $B), the filehandle $A will not have the same file descriptor as $B, and therefore flock($A) will not flock($B) nor vice versa. But with open(my $A, ">>&=", $B), the filehandles will share the same underlying system file descriptor.

Note that under Perls older than 5.8.0, Perl uses the standard C library's' fdopen(3) to implement the = functionality. On many Unix systems, fdopen(3) fails when file descriptors exceed a certain value, typically 255. For Perls 5.8.0 and later, PerlIO is (most often) the default.

You can see whether your Perl was built with PerlIO by running perl -V:useperlio. If it says 'define', you have PerlIO; otherwise you don't.

If you open a pipe on the command - (that is, specify either |- or -| with the one- or two-argument forms of open), an implicit fork is done, so open returns twice: in the parent process it returns the pid of the child process, and in the child process it returns (a defined) 0. Use defined($pid) or // to determine whether the open was successful.

For example, use either

my $child_pid = open(my $from_kid, "-|") // die "Can't fork: $!";

my $child_pid = open(my $to_kid,   "|-") // die "Can't fork: $!";

followed by

    if ($child_pid) {
	# am the parent:
	# either write $to_kid or else read $from_kid
	...
       waitpid $child_pid, 0;
    } else {
	# am the child; use STDIN/STDOUT normally
	...
	exit;
    }

The filehandle behaves normally for the parent, but I/O to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process, the filehandle isn't opened--I/O happens from/to the new STDOUT/STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when running setuid and you don't want to have to scan shell commands for metacharacters.

The following blocks are more or less equivalent:

open(my $fh, "|tr '[a-z]' '[A-Z]'");
open(my $fh, "|-", "tr '[a-z]' '[A-Z]'");
open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(my $fh, "|-", "tr", '[a-z]', '[A-Z]');

open(my $fh, "cat -n '$file'|");
open(my $fh, "-|", "cat -n '$file'");
open(my $fh, "-|") || exec "cat", "-n", $file;
open(my $fh, "-|", "cat", "-n", $file);

The last two examples in each block show the pipe as "list form", which is not yet supported on all platforms. A good rule of thumb is that if your platform has a real fork (in other words, if your platform is Unix, including Linux and MacOS X), you can use the list form. You would want to use the list form of the pipe so you can pass literal arguments to the command without risk of the shell interpreting any shell metacharacters in them. However, this also bars you from opening pipes to commands that intentionally contain shell metacharacters, such as:

    open(my $fh, "|cat -n | expand -4 | lpr")
	|| die "Can't open pipeline to lpr: $!";

See "Safe Pipe Opens" in perlipc for more examples of this.

Perl will attempt to flush all files opened for output before any operation that may do a fork, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the autoflush method of IO::Handle on any open handles.

On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor as determined by the value of $^F. See "$^F" in perlvar.

Closing any piped filehandle causes the parent process to wait for the child to finish, then returns the status value in $? and ${^CHILD_ERROR_NATIVE}.

The filename passed to the one- and two-argument forms of open will have leading and trailing whitespace deleted and normal redirection characters honored. This property, known as "magic open", can often be used to good effect. A user could specify a filename of "rsh cat file |", or you could change certain filenames as needed:

$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(my $fh, $filename) or die "Can't open $filename: $!";

Use the three-argument form to open a file with arbitrary weird characters in it,

    open(my $fh, "<", $file)
	|| die "Can't open $file: $!";

otherwise it's necessary to protect any leading and trailing whitespace:

    $file =~ s#^(\s)#./$1#;
    open(my $fh, "< $file\0")
	|| die "Can't open $file: $!";

(this may not work on some bizarre filesystems). One should conscientiously choose between the magic and three-argument form of open:

open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!";

will allow the user to specify an argument of the form "rsh cat file |", but will not work on a filename that happens to have a trailing space, while

    open(my $in, "<", $ARGV[0])
	|| die "Can't open $ARGV[0]: $!";

will have exactly the opposite restrictions. (However, some shells support the syntax perl your_program.pl <( rsh cat file ), which produces a filename that can be opened normally.)

If you want a "real" C open(2), then you should use the sysopen function, which involves no such magic (but uses different filemodes than Perl open, which corresponds to C fopen(3)). This is another way to protect your filenames from interpretation. For example:

use IO::Handle;
sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL)
    or die "Can't open $path: $!";
$fh->autoflush(1);
print $fh "stuff $$\n";
seek($fh, 0, 0);
print "File contains: ", readline($fh);

See seek for some details about mixing reading and writing.

Portability issues: "open" in perlport.

opendir DIRHANDLE,EXPR

Opens a directory named EXPR for processing by readdir, telldir, seekdir, rewinddir, and closedir. Returns true if successful. DIRHANDLE may be an expression whose value can be used as an indirect dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined scalar variable (or array or hash element), the variable is assigned a reference to a new anonymous dirhandle; that is, it's autovivified. DIRHANDLEs have their own namespace separate from FILEHANDLEs.

See the example at readdir.

ord EXPR

ord

Returns the numeric value of the first character of EXPR. If EXPR is an empty string, returns 0. If EXPR is omitted, uses $_. (Note character, not byte.)

For the reverse, see chr. See perlunicode for more about Unicode.

our VARLIST

our TYPE VARLIST

our VARLIST : ATTRS

our TYPE VARLIST : ATTRS

our makes a lexical alias to a package (i.e. global) variable of the same name in the current package for use within the current lexical scope.

our has the same scoping rules as my or state, meaning that it is only valid within a lexical scope. Unlike my and state, which both declare new (lexical) variables, our only creates an alias to an existing variable: a package variable of the same name.

This means that when use strict 'vars' is in effect, our lets you use a package variable without qualifying it with the package name, but only within the lexical scope of the our declaration. This applies immediately--even within the same statement.

package Foo;
use strict;

$Foo::foo = 23;

{
    our $foo;   # alias to $Foo::foo
    print $foo; # prints 23
}

print $Foo::foo; # prints 23

print $foo; # ERROR: requires explicit package name

This works even if the package variable has not been used before, as package variables spring into existence when first used.

package Foo;
use strict;

our $foo = 23;   # just like $Foo::foo = 23

print $Foo::foo; # prints 23

Because the variable becomes legal immediately under use strict 'vars', so long as there is no variable with that name is already in scope, you can then reference the package variable again even within the same statement.

package Foo;
use strict;

my  $foo = $foo; # error, undeclared $foo on right-hand side
our $foo = $foo; # no errors

If more than one variable is listed, the list must be placed in parentheses.

our($bar, $baz);

An our declaration declares an alias for a package variable that will be visible across its entire lexical scope, even across package boundaries. The package in which the variable is entered is determined at the point of the declaration, not at the point of use. This means the following behavior holds:

package Foo;
our $bar;      # declares $Foo::bar for rest of lexical scope
$bar = 20;

package Bar;
print $bar;    # prints 20, as it refers to $Foo::bar

Multiple our declarations with the same name in the same lexical scope are allowed if they are in different packages. If they happen to be in the same package, Perl will emit warnings if you have asked for them, just like multiple my declarations. Unlike a second my declaration, which will bind the name to a fresh variable, a second our declaration in the same package, in the same scope, is merely redundant.

use warnings;
package Foo;
our $bar;      # declares $Foo::bar for rest of lexical scope
$bar = 20;

package Bar;
our $bar = 30; # declares $Bar::bar for rest of lexical scope
print $bar;    # prints 30

our $bar;      # emits warning but has no other effect
print $bar;    # still prints 30

An our declaration may also have a list of attributes associated with it.

The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound to the use of the fields pragma, and attributes are handled using the attributes pragma, or, starting from Perl 5.8.0, also via the Attribute::Handlers module. See "Private Variables via my()" in perlsub for details.

Note that with a parenthesised list, undef can be used as a dummy placeholder, for example to skip assignment of initial values:

our ( undef, $min, $hour ) = localtime;

our differs from use vars, which allows use of an unqualified name only within the affected package, but across scopes.

pack TEMPLATE,LIST

Takes a LIST of values and converts it into a string using the rules given by the TEMPLATE. The resulting string is the concatenation of the converted values. Typically, each converted value looks like its machine-level representation. For example, on 32-bit machines an integer may be represented by a sequence of 4 bytes, which will in Perl be presented as a string that's 4 characters long.

See perlpacktut for an introduction to this function.

The TEMPLATE is a sequence of characters that give the order and type of values, as follows:

a  A string with arbitrary binary data, will be null padded.
A  A text (ASCII) string, will be space padded.
Z  A null-terminated (ASCIZ) string, will be null padded.

b  A bit string (ascending bit order inside each byte,
   like vec()).
B  A bit string (descending bit order inside each byte).
h  A hex string (low nybble first).
H  A hex string (high nybble first).

c  A signed char (8-bit) value.
C  An unsigned char (octet) value.
W  An unsigned char value (can be greater than 255).

s  A signed short (16-bit) value.
S  An unsigned short value.

l  A signed long (32-bit) value.
L  An unsigned long value.

q  A signed quad (64-bit) value.
Q  An unsigned quad value.
     (Quads are available only if your system supports 64-bit
      integer values _and_ if Perl has been compiled to support
      those.  Raises an exception otherwise.)

i  A signed integer value.
I  A unsigned integer value.
     (This 'integer' is _at_least_ 32 bits wide.  Its exact
      size depends on what a local C compiler calls 'int'.)

n  An unsigned short (16-bit) in "network" (big-endian) order.
N  An unsigned long (32-bit) in "network" (big-endian) order.
v  An unsigned short (16-bit) in "VAX" (little-endian) order.
V  An unsigned long (32-bit) in "VAX" (little-endian) order.

j  A Perl internal signed integer value (IV).
J  A Perl internal unsigned integer value (UV).

f  A single-precision float in native format.
d  A double-precision float in native format.

F  A Perl internal floating-point value (NV) in native format
D  A float of long-double precision in native format.
     (Long doubles are available only if your system supports
      long double values _and_ if Perl has been compiled to
      support those.  Raises an exception otherwise.
      Note that there are different long double formats.)

p  A pointer to a null-terminated string.
P  A pointer to a structure (fixed-length string).

u  A uuencoded string.
U  A Unicode character number.  Encodes to a character in char-
   acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
   byte mode.

w  A BER compressed integer (not an ASN.1 BER, see perlpacktut
   for details).  Its bytes represent an unsigned integer in
   base 128, most significant digit first, with as few digits
   as possible.  Bit eight (the high bit) is set on each byte
   except the last.

x  A null byte (a.k.a ASCII NUL, "\000", chr(0))
X  Back up a byte.
@  Null-fill or truncate to absolute position, counted from the
   start of the innermost ()-group.
.  Null-fill or truncate to absolute position specified by
   the value.
(  Start of a ()-group.

One or more modifiers below may optionally follow certain letters in the TEMPLATE (the second column lists letters for which the modifier is valid):

!   sSlLiI     Forces native (short, long, int) sizes instead
               of fixed (16-/32-bit) sizes.

!   xX         Make x and X act as alignment commands.

!   nNvV       Treat integers as signed instead of unsigned.

!   @.         Specify position as byte offset in the internal
               representation of the packed string.  Efficient
               but dangerous.

>   sSiIlLqQ   Force big-endian byte-order on the type.
    jJfFdDpP   (The "big end" touches the construct.)

<   sSiIlLqQ   Force little-endian byte-order on the type.
    jJfFdDpP   (The "little end" touches the construct.)

The > and < modifiers can also be used on () groups to force a particular byte-order on all components in that group, including all its subgroups.

The following rules apply:

Each letter may optionally be followed by a number indicating the repeat count. A numeric repeat count may optionally be enclosed in brackets, as in pack("C[80]", @arr). The repeat count gobbles that many values from the LIST when used with all format types other than a, A, Z, b, B, h, H, @, ., x, X, and P, where it means something else, described below. Supplying a * for the repeat count instead of a number means to use however many items are left, except for:
- @, x, and X, where it is equivalent to 0.
- <.>, where it means relative to the start of the string.
- u, where it is equivalent to 1 (or 45, which here is equivalent).
One can replace a numeric repeat count with a template letter enclosed in brackets to use the packed byte length of the bracketed template for the repeat count.

For example, the template x[L] skips as many bytes as in a packed long, and the template "$t X[$t] $t" unpacks twice whatever $t (when variable-expanded) unpacks. If the template in brackets contains alignment commands (such as x![d]), its packed length is calculated as if the start of the template had the maximal possible alignment.

When used with Z, a * as the repeat count is guaranteed to add a trailing null byte, so the resulting string is always one byte longer than the byte length of the item itself.

When used with @, the repeat count represents an offset from the start of the innermost () group.

When used with ., the repeat count determines the starting position to calculate the value offset as follows:
- If the repeat count is 0, it's relative to the current position.
- If the repeat count is *, the offset is relative to the start of the packed string.
- And if it's an integer n, the offset is relative to the start of the nth innermost ( ) group, or to the start of the string if n is bigger then the group level.
The repeat count for u is interpreted as the maximal number of bytes to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat count should not be more than 65.
The a, A, and Z types gobble just one value, but pack it as a string of length count, padding with nulls or spaces as needed. When unpacking, A strips trailing whitespace and nulls, Z strips everything after the first null, and a returns data with no stripping at all.

If the value to pack is too long, the result is truncated. If it's too long and an explicit count is provided, Z packs only $count-1 bytes, followed by a null byte. Thus Z always packs a trailing null, except when the count is 0.
Likewise, the b and B formats pack a string that's that many bits long. Each such format generates 1 bit of the result. These are typically followed by a repeat count like B8 or B64.

Each result bit is based on the least-significant bit of the corresponding input character, i.e., on ord($char)%2. In particular, characters "0" and "1" generate bits 0 and 1, as do characters "\000" and "\001".

Starting from the beginning of the input string, each 8-tuple of characters is converted to 1 character of output. With format b, the first character of the 8-tuple determines the least-significant bit of a character; with format B, it determines the most-significant bit of a character.

If the length of the input string is not evenly divisible by 8, the remainder is packed as if the input string were padded by null characters at the end. Similarly during unpacking, "extra" bits are ignored.

If the input string is longer than needed, remaining characters are ignored.

A * for the repeat count uses all characters of the input field. On unpacking, bits are converted to a string of 0s and 1s.
The h and H formats pack a string that many nybbles (4-bit groups, representable as hexadecimal digits, "0".."9" "a".."f") long.

For each such format, pack generates 4 bits of result. With non-alphabetical characters, the result is based on the 4 least-significant bits of the input character, i.e., on ord($char)%16. In particular, characters "0" and "1" generate nybbles 0 and 1, as do bytes "\000" and "\001". For characters "a".."f" and "A".."F", the result is compatible with the usual hexadecimal digits, so that "a" and "A" both generate the nybble 0xA==10. Use only these specific hex characters with this format.

Starting from the beginning of the template to pack, each pair of characters is converted to 1 character of output. With format h, the first character of the pair determines the least-significant nybble of the output character; with format H, it determines the most-significant nybble.

If the length of the input string is not even, it behaves as if padded by a null character at the end. Similarly, "extra" nybbles are ignored during unpacking.

If the input string is longer than needed, extra characters are ignored.

A * for the repeat count uses all characters of the input field. For unpack, nybbles are converted to a string of hexadecimal digits.
The p format packs a pointer to a null-terminated string. You are responsible for ensuring that the string is not a temporary value, as that could potentially get deallocated before you got around to using the packed result. The P format packs a pointer to a structure of the size indicated by the length. A null pointer is created if the corresponding value for p or P is undef; similarly with unpack, where a null pointer unpacks into undef.

If your system has a strange pointer size--meaning a pointer is neither as big as an int nor as big as a long--it may not be possible to pack or unpack pointers in big- or little-endian byte order. Attempting to do so raises an exception.
The / template character allows packing and unpacking of a sequence of items where the packed structure contains a packed item count followed by the packed items themselves. This is useful when the structure you're unpacking has encoded the sizes or repeat counts for some of its fields within the structure itself as separate fields.

For pack, you write length-item/sequence-item, and the length-item describes how the length value is packed. Formats likely to be of most use are integer-packing ones like n for Java strings, w for ASN.1 or SNMP, and N for Sun XDR.

For pack, sequence-item may have a repeat count, in which case the minimum of that and the number of available items is used as the argument for length-item. If it has no repeat count or uses a '*', the number of available items is used.

For unpack, an internal stack of integer arguments unpacked so far is used. You write /sequence-item and the repeat count is obtained by popping off the last element from the stack. The sequence-item must not have a repeat count.

If sequence-item refers to a string type ("A", "a", or "Z"), the length-item is the string length, not the number of strings. With an explicit repeat count for pack, the packed string is adjusted to that length. For example:
```
This code:                             gives this result:

unpack("W/a", "\004Gurusamy")          ("Guru")
unpack("a3/A A*", "007 Bond  J ")      (" Bond", "J")
unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")

pack("n/a* w/a","hello,","world")     "\000\006hello,\005world"
pack("a/W2", ord("a") .. ord("z"))    "2ab"
```
The length-item is not returned explicitly from unpack.

Supplying a count to the length-item format letter is only useful with A, a, or Z. Packing with a length-item of a or Z may introduce "\000" characters, which Perl does not regard as legal in numeric strings.
The integer types s, S, l, and L may be followed by a ! modifier to specify native shorts or longs. As shown in the example above, a bare l means exactly 32 bits, although the native long as seen by the local C compiler may be larger. This is mainly an issue on 64-bit platforms. You can see whether using ! makes any difference this way:
```
    printf "format s is %d, s! is %d\n",
	length pack("s"), length pack("s!");

    printf "format l is %d, l! is %d\n",
	length pack("l"), length pack("l!");
```
i! and I! are also allowed, but only for completeness' sake: they are identical to i and I.

The actual sizes (in bytes) of native shorts, ints, longs, and long longs on the platform where Perl was built are also available from the command line:
```
$ perl -V:{short,int,long{,long}}size
shortsize='2';
intsize='4';
longsize='4';
longlongsize='8';
```
or programmatically via the Config module:
```
use Config;
print $Config{shortsize},    "\n";
print $Config{intsize},      "\n";
print $Config{longsize},     "\n";
print $Config{longlongsize}, "\n";
```
$Config{longlongsize} is undefined on systems without long long support.
The integer formats s, S, i, I, l, L, j, and J are inherently non-portable between processors and operating systems because they obey native byteorder and endianness. For example, a 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively (arranged in and handled by the CPU registers) into bytes as
```
0x12 0x34 0x56 0x78  # big-endian
0x78 0x56 0x34 0x12  # little-endian
```
Basically, Intel and VAX CPUs are little-endian, while everybody else, including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used) them in little-endian mode, but SGI/Cray uses them in big-endian mode.

The names big-endian and little-endian are comic references to the egg-eating habits of the little-endian Lilliputians and the big-endian Blefuscudians from the classic Jonathan Swift satire, Gulliver's Travels. This entered computer lingo via the paper "On Holy Wars and a Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.

Some systems may have even weirder byte orders such as
```
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
```
These are called mid-endian, middle-endian, mixed-endian, or just weird.

You can determine your system endianness with this incantation:
```
printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
```
The byteorder on the platform where Perl was built is also available via Config:
```
use Config;
print "$Config{byteorder}\n";
```
or from the command line:
```
$ perl -V:byteorder
```
Byteorders "1234" and "12345678" are little-endian; "4321" and "87654321" are big-endian. Systems with multiarchitecture binaries will have "ffff", signifying that static information doesn't work, one must use runtime probing.

For portably packed integers, either use the formats n, N, v, and V or else use the > and < modifiers described immediately below. See also perlport.
Also floating point numbers have endianness. Usually (but not always) this agrees with the integer endianness. Even though most platforms these days use the IEEE 754 binary format, there are differences, especially if the long doubles are involved. You can see the Config variables doublekind and longdblkind (also doublesize, longdblsize): the "kind" values are enums, unlike byteorder.

Portability-wise the best option is probably to keep to the IEEE 754 64-bit doubles, and of agreed-upon endianness. Another possibility is the "%a") format of printf.
Starting with Perl 5.10.0, integer and floating-point formats, along with the p and P formats and () groups, may all be followed by the > or < endianness modifiers to respectively enforce big- or little-endian byte-order. These modifiers are especially useful given how n, N, v, and V don't cover signed integers, 64-bit integers, or floating-point values.

Here are some concerns to keep in mind when using an endianness modifier:
- Exchanging signed integers between different platforms works only when all platforms store them in the same format. Most platforms store signed integers in two's-complement notation, so usually this is not an issue.
- The > or < modifiers can only be used on floating-point formats on big- or little-endian machines. Otherwise, attempting to use them raises an exception.
- Forcing big- or little-endian byte-order on floating-point values for data exchange can work only if all platforms use the same binary representation such as IEEE floating-point. Even if all platforms are using IEEE, there may still be subtle differences. Being able to use > or < on floating-point values can be useful, but also dangerous if you don't know exactly what you're doing. It is not a general way to portably store floating-point values.
- When using > or < on a () group, this affects all types inside the group that accept byte-order modifiers, including all subgroups. It is silently ignored for all other types. You are not allowed to override the byte-order within a group that already has a byte-order modifier suffix.
Real numbers (floats and doubles) are in native machine format only. Due to the multiplicity of floating-point formats and the lack of a standard "network" representation for them, no facility for interchange has been made. This means that packed floating-point data written on one machine may not be readable on another, even if both use IEEE floating-point arithmetic (because the endianness of the memory representation is not part of the IEEE spec). See also perlport.

If you know exactly what you're doing, you can use the > or < modifiers to force big- or little-endian byte-order on floating-point values.

Because Perl uses doubles (or long doubles, if configured) internally for all numeric calculation, converting from double into float and thence to double again loses precision, so unpack("f", pack("f", $foo)) will not in general equal $foo.
Pack and unpack can operate in two modes: character mode (C0 mode) where the packed string is processed per character, and UTF-8 byte mode (U0 mode) where the packed string is processed in its UTF-8-encoded Unicode form on a byte-by-byte basis. Character mode is the default unless the format string starts with U. You can always switch mode mid-format with an explicit C0 or U0 in the format. This mode remains in effect until the next mode change, or until the end of the () group it (directly) applies to.

Using C0 to get Unicode characters while using U0 to get non-Unicode bytes is not necessarily obvious. Probably only the first of these is what you want:
```
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
  perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
03B1.03C9
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
  perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
CE.B1.CF.89
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
  perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
CE.B1.CF.89
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
  perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
C3.8E.C2.B1.C3.8F.C2.89
```
Those examples also illustrate that you should not try to use pack/unpack as a substitute for the Encode module.
You must yourself do any alignment or padding by inserting, for example, enough "x"es while packing. There is no way for pack and unpack to know where characters are going to or coming from, so they handle their output and input as flat sequences of characters.
A () group is a sub-TEMPLATE enclosed in parentheses. A group may take a repeat count either as postfix, or for unpack, also via the / template character. Within each repetition of a group, positioning with @ starts over at 0. Therefore, the result of
```
pack("@1A((@2A)@3A)", qw[X Y Z])
```
is the string "\0X\0\0YZ".
x and X accept the ! modifier to act as alignment commands: they jump forward or back to the closest position aligned at a multiple of count characters. For example, to pack or unpack a C structure like
```
    struct {
	char   c;    /* one signed, 8-bit character */
	double d;
	char   cc[2];
    }
```
one may need to use the template c x![d] d c[2]. This assumes that doubles must be aligned to the size of double.

For alignment commands, a count of 0 is equivalent to a count of 1; both are no-ops.
n, N, v and V accept the ! modifier to represent signed 16-/32-bit integers in big-/little-endian order. This is portable only when all platforms sharing packed data use the same binary representation for signed integers; for example, when all platforms use two's-complement representation.
Comments can be embedded in a TEMPLATE using # through the end of line. White space can separate pack codes from each other, but modifiers and repeat counts must follow immediately. Breaking complex templates into individual line-by-line components, suitably annotated, can do as much to improve legibility and maintainability of pack/unpack formats as /x can for complicated pattern matches.
If TEMPLATE requires more arguments than pack is given, pack assumes additional "" arguments. If TEMPLATE requires fewer arguments than given, extra arguments are ignored.
Attempting to pack the special floating point values Inf and NaN (infinity, also in negative, and not-a-number) into packed integer values (like "L") is a fatal error. The reason for this is that there simply isn't any sensible mapping for these special values into integers.

Examples:

$foo = pack("WWWW",65,66,67,68);
# foo eq "ABCD"
$foo = pack("W4",65,66,67,68);
# same thing
$foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters.
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters.  You don't get the
# UTF-8 bytes because the U at the start of the format caused
# a switch to U0-mode, so the UTF-8 bytes get joined into
# characters
$foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
# foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
# This is the UTF-8 encoding of the string in the
# previous example

$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"

# NOTE: The examples above featuring "W" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8.  On EBCDIC systems, the first example would be
#      $foo = pack("WWWW",193,194,195,196);

$foo = pack("s2",1,2);
# "\001\000\002\000" on little-endian
# "\000\001\000\002" on big-endian

$foo = pack("a4","abcd","x","y","z");
# "abcd"

$foo = pack("aaaa","abcd","x","y","z");
# "axyz"

$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"

$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)

$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)

@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"

sub bintodec {
    unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}

$foo = pack('sx2l', 12, 34);
# short 12, two zero bytes padding, long 34
$bar = pack('s@4l', 12, 34);
# short 12, zero fill to position 4, long 34
# $foo eq $bar
$baz = pack('s.l', 12, 4, 34);
# short 12, zero fill to position 4, long 34

$foo = pack('nN', 42, 4711);
# pack big-endian 16- and 32-bit unsigned integers
$foo = pack('S>L>', 42, 4711);
# exactly the same
$foo = pack('s<l<', -42, 4711);
# pack little-endian 16- and 32-bit signed integers
$foo = pack('(sl)<', -42, 4711);
# exactly the same

The same template may generally also be used in unpack.

package NAMESPACE

package NAMESPACE VERSION

package NAMESPACE BLOCK

package NAMESPACE VERSION BLOCK

Declares the BLOCK or the rest of the compilation unit as being in the given namespace. The scope of the package declaration is either the supplied code BLOCK or, in the absence of a BLOCK, from the declaration itself through the end of current scope (the enclosing block, file, or eval). That is, the forms without a BLOCK are operative through the end of the current scope, just like the my, state, and our operators. All unqualified dynamic identifiers in this scope will be in the given namespace, except where overridden by another package declaration or when they're one of the special identifiers that qualify into main::, like STDOUT, ARGV, ENV, and the punctuation variables.

A package statement affects dynamic variables only, including those you've used local on, but not lexically-scoped variables, which are created with my, state, or our. Typically it would be the first declaration in a file included by require or use. You can switch into a package in more than one place, since this only determines which default symbol table the compiler uses for the rest of that block. You can refer to identifiers in other packages than the current one by prefixing the identifier with the package name and a double colon, as in $SomePack::var or ThatPack::INPUT_HANDLE. If package name is omitted, the main package as assumed. That is, $::sail is equivalent to $main::sail (as well as to $main'sail, still seen in ancient code, mostly from Perl 4).

If VERSION is provided, package sets the $VERSION variable in the given namespace to a version object with the VERSION provided. VERSION must be a "strict" style version number as defined by the version module: a positive decimal number (integer or decimal-fraction) without exponentiation or else a dotted-decimal v-string with a leading 'v' character and at least three components. You should set $VERSION only once per package.

See "Packages" in perlmod for more information about packages, modules, and classes. See perlsub for other scoping issues.

__PACKAGE__

A special token that returns the name of the package in which it occurs.

pipe READHANDLE,WRITEHANDLE

Opens a pair of connected pipes like the corresponding system call. Note that if you set up a loop of piped processes, deadlock can occur unless you are very careful. In addition, note that Perl's pipes use IO buffering, so you may need to set $| to flush your WRITEHANDLE after each command, depending on the application.

Returns true on success.

See IPC::Open2, IPC::Open3, and "Bidirectional Communication with Another Process" in perlipc for examples of such things.

On systems that support a close-on-exec flag on files, that flag is set on all newly opened file descriptors whose filenos are higher than the current value of $^F (by default 2 for STDERR). See "$^F" in perlvar.

pop ARRAY

pop

Pops and returns the last value of the array, shortening the array by one element.

Returns the undefined value if the array is empty, although this may also happen at other times. If ARRAY is omitted, pops the @ARGV array in the main program, but the @_ array in subroutines, just like shift.

Starting with Perl 5.14, an experimental feature allowed pop to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.

pos SCALAR

pos

Returns the offset of where the last m//g search left off for the variable in question ($_ is used when the variable is not specified). Note that 0 is a valid match offset. undef indicates that the search position is reset (usually due to match failure, but can also be because no match has yet been run on the scalar).

pos directly accesses the location used by the regexp engine to store the offset, so assigning to pos will change that offset, and so will also influence the \G zero-width assertion in regular expressions. Both of these effects take place for the next match, so you can't affect the position with pos during the current match, such as in (?{pos() = 5}) or s//pos() = 5/e.

Setting pos also resets the matched with zero-length flag, described under "Repeated Patterns Matching a Zero-length Substring" in perlre.

Because a failed m//gc match doesn't reset the offset, the return from pos won't change either in this case. See perlre and perlop.

print FILEHANDLE LIST

print FILEHANDLE

print LIST

print

Prints a string or a list of strings. Returns true if successful. FILEHANDLE may be a scalar variable containing the name of or a reference to the filehandle, thus introducing one level of indirection. (NOTE: If FILEHANDLE is a variable and the next token is a term, it may be misinterpreted as an operator unless you interpose a + or put parentheses around the arguments.) If FILEHANDLE is omitted, prints to the last selected (see select) output handle. If LIST is omitted, prints $_ to the currently selected output handle. To use FILEHANDLE alone to print the content of $_ to it, you must use a bareword filehandle like FH, not an indirect one like $fh. To set the default output handle to something other than STDOUT, use the select operation.

The current value of $, (if any) is printed between each LIST item. The current value of $\ (if any) is printed after the entire LIST has been printed. Because print takes a LIST, anything in the LIST is evaluated in list context, including any subroutines whose return lists you pass to print. Be careful not to follow the print keyword with a left parenthesis unless you want the corresponding right parenthesis to terminate the arguments to the print; put parentheses around all arguments (or interpose a +, but that doesn't look as good).

If you're storing handles in an array or hash, or in general whenever you're using any expression more complex than a bareword handle or a plain, unsubscripted scalar variable to retrieve it, you will have to use a block returning the filehandle value instead, in which case the LIST may not be omitted:

print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";

Printing to a closed pipe or socket will generate a SIGPIPE signal. See perlipc for more on signal handling.

printf FILEHANDLE FORMAT, LIST

printf FILEHANDLE

printf FORMAT, LIST

printf

Equivalent to print FILEHANDLE sprintf(FORMAT, LIST), except that $\ (the output record separator) is not appended. The FORMAT and the LIST are actually parsed as a single list. The first argument of the list will be interpreted as the printf format. This means that printf(@_) will use $_[0] as the format. See sprintf for an explanation of the format argument. If use locale (including use locale ':not_characters') is in effect and POSIX::setlocale has been called, the character used for the decimal separator in formatted floating-point numbers is affected by the LC_NUMERIC locale setting. See perllocale and POSIX.

For historical reasons, if you omit the list, $_ is used as the format; to use FILEHANDLE without a list, you must use a bareword filehandle like FH, not an indirect one like $fh. However, this will rarely do what you want; if $_ contains formatting codes, they will be replaced with the empty string and a warning will be emitted if warnings are enabled. Just use print if you want to print the contents of $_.

Don't fall into the trap of using a printf when a simple print would do. The print is more efficient and less error prone.

prototype FUNCTION

prototype

Returns the prototype of a function as a string (or undef if the function has no prototype). FUNCTION is a reference to, or the name of, the function whose prototype you want to retrieve. If FUNCTION is omitted, $_ is used.

If FUNCTION is a string starting with CORE::, the rest is taken as a name for a Perl builtin. If the builtin's arguments cannot be adequately expressed by a prototype (such as system), prototype returns undef, because the builtin does not really behave like a Perl function. Otherwise, the string describing the equivalent prototype is returned.

push ARRAY,LIST

Treats ARRAY as a stack by appending the values of LIST to the end of ARRAY. The length of ARRAY increases by the length of LIST. Has the same effect as

for my $value (LIST) {
    $ARRAY[++$#ARRAY] = $value;
}

but is more efficient. Returns the number of elements in the array following the completed push.

Starting with Perl 5.14, an experimental feature allowed push to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.

q/STRING/

qq/STRING/

qw/STRING/

qx/STRING/

Generalized quotes. See "Quote-Like Operators" in perlop.

qr/STRING/

Regexp-like quote. See "Regexp Quote-Like Operators" in perlop.

quotemeta EXPR

quotemeta

Returns the value of EXPR with all the ASCII non-"word" characters backslashed. (That is, all ASCII characters not matching /[A-Za-z_0-9]/ will be preceded by a backslash in the returned string, regardless of any locale settings.) This is the internal function implementing the \Q escape in double-quoted strings. (See below for the behavior on non-ASCII code points.)

If EXPR is omitted, uses $_.

quotemeta (and \Q ... \E) are useful when interpolating strings into regular expressions, because by default an interpolated variable will be considered a mini-regular expression. For example:

my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
$sentence =~ s{$substring}{big bad wolf};

Will cause $sentence to become 'The big bad wolf jumped over...'.

On the other hand:

my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
$sentence =~ s{\Q$substring\E}{big bad wolf};

Or:

my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
my $quoted_substring = quotemeta($substring);
$sentence =~ s{$quoted_substring}{big bad wolf};

Will both leave the sentence as is. Normally, when accepting literal string input from the user, quotemeta or \Q must be used.

In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded strings, but not quoted in UTF-8 strings.

Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for quoting non-ASCII characters; the quoting of ASCII characters is unchanged.

Also unchanged is the quoting of non-UTF-8 strings when outside the scope of a use feature 'unicode_strings', which is to quote all characters in the upper Latin1 range. This provides complete backwards compatibility for old programs which do not use Unicode. (Note that unicode_strings is automatically enabled within the scope of a use v5.12 or greater.)

Within the scope of use locale, all non-ASCII Latin1 code points are quoted whether the string is encoded as UTF-8 or not. As mentioned above, locale does not affect the quoting of ASCII-range characters. This protects against those locales where characters such as "|" are considered to be word characters.

Otherwise, Perl quotes non-ASCII characters using an adaptation from Unicode (see http://www.unicode.org/reports/tr31/). The only code points that are quoted are those that have any of the Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space, Default_Ignorable_Code_Point, or General_Category=Control.

Of these properties, the two important ones are Pattern_Syntax and Pattern_White_Space. They have been set up by Unicode for exactly this purpose of deciding which characters in a regular expression pattern should be quoted. No character that can be in an identifier has these properties.

Perl promises, that if we ever add regular expression pattern metacharacters to the dozen already defined (\ | ( ) [ { ^ $ * + ? .), that we will only use ones that have the Pattern_Syntax property. Perl also promises, that if we ever add characters that are considered to be white space in regular expressions (currently mostly affected by /x), they will all have the Pattern_White_Space property.

Unicode promises that the set of code points that have these two properties will never change, so something that is not quoted in v5.16 will never need to be quoted in any future Perl release. (Not all the code points that match Pattern_Syntax have actually had characters assigned to them; so there is room to grow, but they are quoted whether assigned or not. Perl, of course, would never use an unassigned code point as an actual metacharacter.)

Quoting characters that have the other 3 properties is done to enhance the readability of the regular expression and not because they actually need to be quoted for regular expression purposes (characters with the White_Space property are likely to be indistinguishable on the page or screen from those with the Pattern_White_Space property; and the other two properties contain non-printing characters).

rand EXPR

rand

Returns a random fractional number greater than or equal to 0 and less than the value of EXPR. (EXPR should be positive.) If EXPR is omitted, the value 1 is used. Currently EXPR with the value 0 is also special-cased as 1 (this was undocumented before Perl 5.8.0 and is subject to change in future versions of Perl). Automatically calls srand unless srand has already been called. See also srand.

Apply int to the value returned by rand if you want random integers instead of random fractional numbers. For example,

int(rand(10))

returns a random integer between 0 and 9, inclusive.

(Note: If your rand function consistently returns numbers that are too large or too small, then your version of Perl was probably compiled with the wrong number of RANDBITS.)

rand is not cryptographically secure. You should not rely on it in security-sensitive situations. As of this writing, a number of third-party CPAN modules offer random number generators intended by their authors to be cryptographically secure, including: Data::Entropy, Crypt::Random, Math::Random::Secure, and Math::TrulyRandom.

read FILEHANDLE,SCALAR,LENGTH,OFFSET

read FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH characters of data into variable SCALAR from the specified FILEHANDLE. Returns the number of characters actually read, 0 at end of file, or undef if there was an error (in the latter case $! is also set). SCALAR will be grown or shrunk so that the last character actually read is the last character of the scalar after the read.

An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many characters counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with "\0" bytes before the result of the read is appended.

The call is implemented in terms of either Perl's or your system's native fread(3) library function. To get a true read(2) system call, see sysread.

Note the characters: depending on the status of the filehandle, either (8-bit) bytes or characters are read. By default, all filehandles operate on bytes, but for example if the filehandle has been opened with the :utf8 I/O layer (see open, and the open pragma), the I/O will operate on UTF8-encoded Unicode characters, not bytes. Similarly for the :encoding layer: in that case pretty much any characters can be read.

readdir DIRHANDLE

Returns the next directory entry for a directory opened by opendir. If used in list context, returns all the rest of the entries in the directory. If there are no more entries, returns the undefined value in scalar context and the empty list in list context.

If you're planning to filetest the return values out of a readdir, you'd better prepend the directory in question. Otherwise, because we didn't chdir there, it would have been testing the wrong file.

opendir(my $dh, $some_dir) || die "Can't opendir $some_dir: $!";
my @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
closedir $dh;

As of Perl 5.12 you can use a bare readdir in a while loop, which will set $_ on every iteration.

opendir(my $dh, $some_dir) || die "Can't open $some_dir: $!";
while (readdir $dh) {
    print "$some_dir/$_\n";
}
closedir $dh;

To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious failures, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:

use 5.012; # so readdir assigns to $_ in a lone while test

readline EXPR

readline

Reads from the filehandle whose typeglob is contained in EXPR (or from *ARGV if EXPR is not provided). In scalar context, each call reads and returns the next line until end-of-file is reached, whereupon the subsequent call returns undef. In list context, reads until end-of-file is reached and returns a list of lines. Note that the notion of "line" used here is whatever you may have defined with $/ (or $INPUT_RECORD_SEPARATOR in English). See "$/" in perlvar.

When $/ is set to undef, when readline is in scalar context (i.e., file slurp mode), and when an empty file is read, it returns '' the first time, followed by undef subsequently.

This is the internal function implementing the <EXPR> operator, but you can use it directly. The <EXPR> operator is discussed in more detail in "I/O Operators" in perlop.

my $line = <STDIN>;
my $line = readline(STDIN);    # same thing

If readline encounters an operating system error, $! will be set with the corresponding error message. It can be helpful to check $! when you are reading from filehandles you don't trust, such as a tty or a socket. The following example uses the operator form of readline and dies if the result is not defined.

while ( ! eof($fh) ) {
    defined( $_ = readline $fh ) or die "readline failed: $!";
    ...
}

Note that you have can't handle readline errors that way with the ARGV filehandle. In that case, you have to open each element of @ARGV yourself since eof handles ARGV differently.

foreach my $arg (@ARGV) {
    open(my $fh, $arg) or warn "Can't open $arg: $!";

    while ( ! eof($fh) ) {
        defined( $_ = readline $fh )
            or die "readline failed for $arg: $!";
        ...
    }
}

readlink EXPR

readlink

Returns the value of a symbolic link, if symbolic links are implemented. If not, raises an exception. If there is a system error, returns the undefined value and sets $! (errno). If EXPR is omitted, uses $_.

Portability issues: "readlink" in perlport.

readpipe EXPR

readpipe

EXPR is executed as a system command. The collected standard output of the command is returned. In scalar context, it comes back as a single (potentially multi-line) string. In list context, returns a list of lines (however you've defined lines with $/ (or $INPUT_RECORD_SEPARATOR in English)). This is the internal function implementing the qx/EXPR/ operator, but you can use it directly. The qx/EXPR/ operator is discussed in more detail in "I/O Operators" in perlop. If EXPR is omitted, uses $_.

recv SOCKET,SCALAR,LENGTH,FLAGS

Receives a message on a socket. Attempts to receive LENGTH characters of data into variable SCALAR from the specified SOCKET filehandle. SCALAR will be grown or shrunk to the length actually read. Takes the same flags as the system call of the same name. Returns the address of the sender if SOCKET's protocol supports this; returns an empty string otherwise. If there's an error, returns the undefined value. This call is actually implemented in terms of the recvfrom(2) system call. See "UDP: Message Passing" in perlipc for examples.

Note the characters: depending on the status of the socket, either (8-bit) bytes or characters are received. By default all sockets operate on bytes, but for example if the socket has been changed using binmode to operate with the :encoding(utf8) I/O layer (see the open pragma), the I/O will operate on UTF8-encoded Unicode characters, not bytes. Similarly for the :encoding layer: in that case pretty much any characters can be read.

redo LABEL

redo EXPR

redo

The redo command restarts the loop block without evaluating the conditional again. The continue block, if any, is not executed. If the LABEL is omitted, the command refers to the innermost enclosing loop. The redo EXPR form, available starting in Perl 5.18.0, allows a label name to be computed at run time, and is otherwise identical to redo LABEL. Programs that want to lie to themselves about what was just input normally use this command:

# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
LINE: while (<STDIN>) {
    while (s|({.*}.*){.*}|$1 |) {}
    s|{.*}| |;
    if (s|{.*| |) {
        my $front = $_;
        while (<STDIN>) {
            if (/}/) {  # end of comment?
                s|^|$front\{|;
                redo LINE;
            }
        }
    }
    print;
}

redo cannot be used to retry a block that returns a value such as eval {}, sub {}, or do {}, and should not be used to exit a grep or map operation.

Note that a block by itself is semantically identical to a loop that executes once. Thus redo inside such a block will effectively turn it into a looping construct.

See also continue for an illustration of how last, next, and redo work.

Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so redo ("foo")."bar" will cause "bar" to be part of the argument to redo.

ref EXPR

ref

Returns a non-empty string if EXPR is a reference, the empty string otherwise. If EXPR is not specified, $_ will be used. The value returned depends on the type of thing the reference is a reference to.

Builtin types include:

SCALAR
ARRAY
HASH
CODE
REF
GLOB
LVALUE
FORMAT
IO
VSTRING
Regexp

You can think of ref as a typeof operator.

if (ref($r) eq "HASH") {
    print "r is a reference to a hash.\n";
}
unless (ref($r)) {
    print "r is not a reference at all.\n";
}

The return value LVALUE indicates a reference to an lvalue that is not a variable. You get this from taking the reference of function calls like pos or substr. VSTRING is returned if the reference points to a version string.

The result Regexp indicates that the argument is a regular expression resulting from qr//.

If the referenced object has been blessed into a package, then that package name is returned instead. But don't use that, as it's now considered "bad practice". For one reason, an object could be using a class called Regexp or IO, or even HASH. Also, ref doesn't take into account subclasses, like isa does.

Instead, use blessed (in the Scalar::Util module) for boolean checks, isa for specific class checks and reftype (also from Scalar::Util) for type checks. (See perlobj for details and a blessed/isa example.)

See also perlref.

rename OLDNAME,NEWNAME

Changes the name of a file; an existing file NEWNAME will be clobbered. Returns true for success, false otherwise.

Behavior of this function varies wildly depending on your system implementation. For example, it will usually not work across file system boundaries, even though the system mv command sometimes compensates for this. Other restrictions include whether it works on directories, open files, or pre-existing files. Check perlport and either the rename(2) manpage or equivalent system documentation for details.

For a platform independent move function look at the File::Copy module.

Portability issues: "rename" in perlport.

require VERSION

require EXPR

require

Demands a version of Perl specified by VERSION, or demands some semantics specified by EXPR or by $_ if EXPR is not supplied.

VERSION may be either a numeric argument such as 5.006, which will be compared to $], or a literal of the form v5.6.1, which will be compared to $^V (or $PERL_VERSION in English). An exception is raised if VERSION is greater than the version of the current Perl interpreter. Compare with use, which can do a similar check at compile time.

Specifying VERSION as a literal of the form v5.6.1 should generally be avoided, because it leads to misleading error messages under earlier versions of Perl that do not support this syntax. The equivalent numeric version should be used instead.

require v5.6.1;     # run time version check
require 5.6.1;      # ditto
require 5.006_001;  # ditto; preferred for backwards
                      compatibility

Otherwise, require demands that a library file be included if it hasn't already been included. The file is included via the do-FILE mechanism, which is essentially just a variety of eval with the caveat that lexical variables in the invoking script will be invisible to the included code. If it were implemented in pure Perl, it would have semantics similar to the following:

use Carp 'croak';
use version;

sub require {
    my ($filename) = @_;
    if ( my $version = eval { version->parse($filename) } ) {
        if ( $version > $^V ) {
           my $vn = $version->normal;
           croak "Perl $vn required--this is only $^V, stopped";
        }
        return 1;
    }

    if (exists $INC{$filename}) {
        return 1 if $INC{$filename};
        croak "Compilation failed in require";
    }

    foreach $prefix (@INC) {
        if (ref($prefix)) {
            #... do other stuff - see text below ....
        }
        # (see text below about possible appending of .pmc
        # suffix to $filename)
        my $realfilename = "$prefix/$filename";
        next if ! -e $realfilename || -d _ || -b _;
        $INC{$filename} = $realfilename;
        my $result = do($realfilename);
                     # but run in caller's namespace

        if (!defined $result) {
            $INC{$filename} = undef;
            croak $@ ? "$@Compilation failed in require"
                     : "Can't locate $filename: $!\n";
        }
        if (!$result) {
            delete $INC{$filename};
            croak "$filename did not return true value";
        }
        $! = 0;
        return $result;
    }
    croak "Can't locate $filename in \@INC ...";
}

Note that the file will not be included twice under the same specified name.

The file must return true as the last statement to indicate successful execution of any initialization code, so it's customary to end such a file with 1; unless you're sure it'll return true otherwise. But it's better just to put the 1;, in case you add more statements.

If EXPR is a bareword, require assumes a .pm extension and replaces :: with / in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace.

In other words, if you try this:

require Foo::Bar;     # a splendid bareword

The require function will actually look for the Foo/Bar.pm file in the directories specified in the @INC array.

But if you try this:

    my $class = 'Foo::Bar';
    require $class;       # $class is not a bareword
#or
    require "Foo::Bar";   # not a bareword because of the ""

The require function will look for the Foo::Bar file in the @INC array and will complain about not finding Foo::Bar there. In this case you can do:

eval "require $class";

Now that you understand how require looks for files with a bareword argument, there is a little extra functionality going on behind the scenes. Before require looks for a .pm extension, it will first look for a similar filename with a .pmc extension. If this file is found, it will be loaded in place of any file ending in a .pm extension.

You can also insert hooks into the import facility by putting Perl code directly into the @INC array. There are three forms of hooks: subroutine references, array references, and blessed objects.

Subroutine references are the simplest case. When the inclusion system walks through @INC and encounters a subroutine, this subroutine gets called with two parameters, the first a reference to itself, and the second the name of the file to be included (e.g., Foo/Bar.pm). The subroutine should return either nothing or else a list of up to four values in the following order:

A reference to a scalar, containing any initial source code to prepend to the file or generator output.
A filehandle, from which the file will be read.
A reference to a subroutine. If there is no filehandle (previous item), then this subroutine is expected to generate one line of source code per call, writing the line into $_ and returning 1, then finally at end of file returning 0. If there is a filehandle, then the subroutine will be called to act as a simple source filter, with the line as read in $_. Again, return 1 for each valid line, and 0 after all lines have been returned.
Optional state for the subroutine. The state is passed in as $_[1]. A reference to the subroutine itself is passed in as $_[0].

If an empty list, undef, or nothing that matches the first 3 values above is returned, then require looks at the remaining elements of @INC. Note that this filehandle must be a real filehandle (strictly a typeglob or reference to a typeglob, whether blessed or unblessed); tied filehandles will be ignored and processing will stop there.

If the hook is an array reference, its first element must be a subroutine reference. This subroutine is called as above, but the first parameter is the array reference. This lets you indirectly pass arguments to the subroutine.

In other words, you can write:

push @INC, \&my_sub;
sub my_sub {
    my ($coderef, $filename) = @_;  # $coderef is \&my_sub
    ...
}

or:

push @INC, [ \&my_sub, $x, $y, ... ];
sub my_sub {
    my ($arrayref, $filename) = @_;
    # Retrieve $x, $y, ...
    my (undef, @parameters) = @$arrayref;
    ...
}

If the hook is an object, it must provide an INC method that will be called as above, the first parameter being the object itself. (Note that you must fully qualify the sub's name, as unqualified INC is always forced into package main.) Here is a typical code layout:

# In Foo.pm
package Foo;
sub new { ... }
sub Foo::INC {
    my ($self, $filename) = @_;
    ...
}

# In the main program
push @INC, Foo->new(...);

These hooks are also permitted to set the %INC entry corresponding to the files they have loaded. See "%INC" in perlvar.

For a yet-more-powerful import facility, see use and perlmod.

reset EXPR

reset

Generally used in a continue block at the end of a loop to clear variables and reset m?pattern? searches so that they work again. The expression is interpreted as a list of single characters (hyphens allowed for ranges). All variables and arrays beginning with one of those letters are reset to their pristine state. If the expression is omitted, one-match searches (m?pattern?) are reset to match again. Only resets variables or searches in the current package. Always returns 1. Examples:

reset 'X';      # reset all X variables
reset 'a-z';    # reset lower case variables
reset;          # just reset m?one-time? searches

Resetting "A-Z" is not recommended because you'll wipe out your @ARGV and @INC arrays and your %ENV hash. Resets only package variables; lexical variables are unaffected, but they clean themselves up on scope exit anyway, so you'll probably want to use them instead. See my.

return EXPR

return

Returns from a subroutine, eval, do FILE, sort block or regex eval block (but not a grep or map block) with the value given in EXPR. Evaluation of EXPR may be in list, scalar, or void context, depending on how the return value will be used, and the context may vary from one execution to the next (see wantarray). If no EXPR is given, returns an empty list in list context, the undefined value in scalar context, and (of course) nothing at all in void context.

(In the absence of an explicit return, a subroutine, eval, or do FILE automatically returns the value of the last expression evaluated.)

Unlike most named operators, this is also exempt from the looks-like-a-function rule, so return ("foo")."bar" will cause "bar" to be part of the argument to return.

reverse LIST

In list context, returns a list value consisting of the elements of LIST in the opposite order. In scalar context, concatenates the elements of LIST and returns a string value with all characters in the opposite order.

print join(", ", reverse "world", "Hello"); # Hello, world

print scalar reverse "dlrow ,", "olleH";    # Hello, world

Used without arguments in scalar context, reverse reverses $_.

$_ = "dlrow ,olleH";
print reverse;                         # No output, list context
print scalar reverse;                  # Hello, world

Note that reversing an array to itself (as in @a = reverse @a) will preserve non-existent elements whenever possible; i.e., for non-magical arrays or for tied arrays with EXISTS and DELETE methods.

This operator is also handy for inverting a hash, although there are some caveats. If a value is duplicated in the original hash, only one of those can be represented as a key in the inverted hash. Also, this has to unwind one hash and build a whole new one, which may take some time on a large hash, such as from a DBM file.

my %by_name = reverse %by_address;  # Invert the hash

rewinddir DIRHANDLE

Sets the current position to the beginning of the directory for the readdir routine on DIRHANDLE.

Portability issues: "rewinddir" in perlport.

rindex STR,SUBSTR,POSITION

rindex STR,SUBSTR

Works just like index except that it returns the position of the last occurrence of SUBSTR in STR. If POSITION is specified, returns the last occurrence beginning at or before that position.

rmdir FILENAME

rmdir

Deletes the directory specified by FILENAME if that directory is empty. If it succeeds it returns true; otherwise it returns false and sets $! (errno). If FILENAME is omitted, uses $_.

To remove a directory tree recursively (rm -rf on Unix) look at the rmtree function of the File::Path module.

s///

The substitution operator. See "Regexp Quote-Like Operators" in perlop.

say FILEHANDLE LIST

say FILEHANDLE

say LIST

say

Just like print, but implicitly appends a newline. say LIST is simply an abbreviation for { local $\ = "\n"; print LIST }. To use FILEHANDLE without a LIST to print the contents of $_ to it, you must use a bareword filehandle like FH, not an indirect one like $fh.

scalar EXPR

Forces EXPR to be interpreted in scalar context and returns the value of EXPR.

my @counts = ( scalar @a, scalar @b, scalar @c );

There is no equivalent operator to force an expression to be interpolated in list context because in practice, this is never needed. If you really wanted to do so, however, you could use the construction @{[ (some expression) ]}, but usually a simple (some expression) suffices.

Because scalar is a unary operator, if you accidentally use a parenthesized list for the EXPR, this behaves as a scalar comma expression, evaluating all but the last element in void context and returning the final element evaluated in scalar context. This is seldom what you want.

The following single statement:

print uc(scalar(foo(), $bar)), $baz;

is the moral equivalent of these two:

foo();
print(uc($bar), $baz);

See perlop for more details on unary operators and the comma operator, and perldata for details on evaluating a hash in scalar contex.

seek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's position, just like the fseek(3) call of C stdio. FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are 0 to set the new position in bytes to POSITION; 1 to set it to the current position plus POSITION; and 2 to set it to EOF plus POSITION, typically negative. For WHENCE you may use the constants SEEK_SET, SEEK_CUR, and SEEK_END (start of the file, current position, end of the file) from the Fcntl module. Returns 1 on success, false otherwise.

Note the in bytes: even if the filehandle has been set to operate on characters (for example by using the :encoding(utf8) open layer), tell will return byte offsets, not character offsets (because implementing that would render seek and tell rather slow).

If you want to position the file for sysread or syswrite, don't use seek, because buffering makes its effect on the file's read-write position unpredictable and non-portable. Use sysseek instead.

Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever you switch between reading and writing. Amongst other things, this may have the effect of calling stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving the file position:

seek($fh, 0, 1);

This is also useful for applications emulating tail -f. Once you hit EOF on your read and then sleep for a while, you (probably) have to stick in a dummy seek to reset things. The seek doesn't change the position, but it does clear the end-of-file condition on the handle, so that the next readline FILE makes Perl try again to read something. (We hope.)

If that doesn't work (some I/O implementations are particularly cantankerous), you might need something like this:

for (;;) {
    for ($curpos = tell($fh); $_ = readline($fh);
         $curpos = tell($fh)) {
        # search for some stuff and put it into files
    }
    sleep($for_a_while);
    seek($fh, $curpos, 0);
}

seekdir DIRHANDLE,POS

Sets the current position for the readdir routine on DIRHANDLE. POS must be a value returned by telldir. seekdir also has the same caveats about possible directory compaction as the corresponding system library routine.

select FILEHANDLE

select

Returns the currently selected filehandle. If FILEHANDLE is supplied, sets the new current default filehandle for output. This has two effects: first, a write or a print without a filehandle default to this FILEHANDLE. Second, references to variables related to output will refer to this output channel.

For example, to set the top-of-form format for more than one output channel, you might do the following:

select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';

FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:

my $oldfh = select(STDERR); $| = 1; select($oldfh);

Some programmers may prefer to think of filehandles as objects with methods, preferring to write the last example as:

STDERR->autoflush(1);

(Prior to Perl version 5.14, you have to use IO::Handle; explicitly first.)

Portability issues: "select" in perlport.

select RBITS,WBITS,EBITS,TIMEOUT

This calls the select(2) syscall with the bit masks specified, which can be constructed using fileno and vec, along these lines:

my $rin = my $win = my $ein = '';
vec($rin, fileno(STDIN),  1) = 1;
vec($win, fileno(STDOUT), 1) = 1;
$ein = $rin | $win;

If you want to select on many filehandles, you may wish to write a subroutine like this:

sub fhbits {
    my @fhlist = @_;
    my $bits = "";
    for my $fh (@fhlist) {
        vec($bits, fileno($fh), 1) = 1;
    }
    return $bits;
}
my $rin = fhbits(\*STDIN, $tty, $mysock);

The usual idiom is:

my ($nfound, $timeleft) =
  select(my $rout = $rin, my $wout = $win, my $eout = $ein,
                                                         $timeout);

or to block until something becomes ready just do this

my $nfound =
  select(my $rout = $rin, my $wout = $win, my $eout = $ein, undef);

Most systems do not bother to return anything useful in $timeleft, so calling select in scalar context just returns $nfound.

Any of the bit masks can also be undef. The timeout, if specified, is in seconds, which may be fractional. Note: not all implementations are capable of returning the $timeleft. If not, they always return $timeleft equal to the supplied $timeout.

You can effect a sleep of 250 milliseconds this way:

select(undef, undef, undef, 0.25);

Note that whether select gets restarted after signals (say, SIGALRM) is implementation-dependent. See also perlport for notes on the portability of select.

On error, select behaves just like select(2): it returns -1 and sets $!.

On some Unixes, select(2) may report a socket file descriptor as "ready for reading" even when no data is available, and thus any subsequent read would block. This can be avoided if you always use O_NONBLOCK on the socket. See select(2) and fcntl(2) for further details.

The standard IO::Select module provides a user-friendlier interface to select, mostly because it does all the bit-mask work for you.

WARNING: One should not attempt to mix buffered I/O (like read or readline) with select, except as permitted by POSIX, and even then only on POSIX systems. You have to use sysread instead.

Portability issues: "select" in perlport.

semctl ID,SEMNUM,CMD,ARG

Calls the System V IPC function semctl(2). You'll probably have to say

use IPC::SysV;

first to get the correct constant definitions. If CMD is IPC_STAT or GETALL, then ARG must be a variable that will hold the returned semid_ds structure or semaphore value array. Returns like ioctl: the undefined value for error, "0 but true" for zero, or the actual return value otherwise. The ARG must consist of a vector of native short integers, which may be created with pack("s!",(0)x$nsem). See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Semaphore.

Portability issues: "semctl" in perlport.

semget KEY,NSEMS,FLAGS

Calls the System V IPC function semget(2). Returns the semaphore id, or the undefined value on error. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Semaphore.

Portability issues: "semget" in perlport.

semop KEY,OPSTRING

Calls the System V IPC function semop(2) for semaphore operations such as signalling and waiting. OPSTRING must be a packed array of semop structures. Each semop structure can be generated with pack("s!3", $semnum, $semop, $semflag). The length of OPSTRING implies the number of semaphore operations. Returns true if successful, false on error. As an example, the following code waits on semaphore $semnum of semaphore id $semid:

my $semop = pack("s!3", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);

To signal the semaphore, replace -1 with 1. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Semaphore.

Portability issues: "semop" in perlport.

send SOCKET,MSG,FLAGS,TO

send SOCKET,MSG,FLAGS

Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET filehandle. Takes the same flags as the system call of the same name. On unconnected sockets, you must specify a destination to send to, in which case it does a sendto(2) syscall. Returns the number of characters sent, or the undefined value on error. The sendmsg(2) syscall is currently unimplemented. See "UDP: Message Passing" in perlipc for examples.

Note the characters: depending on the status of the socket, either (8-bit) bytes or characters are sent. By default all sockets operate on bytes, but for example if the socket has been changed using binmode to operate with the :encoding(utf8) I/O layer (see open, or the open pragma), the I/O will operate on UTF-8 encoded Unicode characters, not bytes. Similarly for the :encoding layer: in that case pretty much any characters can be sent.

setpgrp PID,PGRP

Sets the current process group for the specified PID, 0 for the current process. Raises an exception when used on a machine that doesn't implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, it defaults to 0,0. Note that the BSD 4.2 version of setpgrp does not accept any arguments, so only setpgrp(0,0) is portable. See also POSIX::setsid().

Portability issues: "setpgrp" in perlport.

setpriority WHICH,WHO,PRIORITY

Sets the current priority for a process, a process group, or a user. (See setpriority(2).) Raises an exception when used on a machine that doesn't implement setpriority(2).

Portability issues: "setpriority" in perlport.

setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL

Sets the socket option requested. Returns undef on error. Use integer constants provided by the Socket module for LEVEL and OPNAME. Values for LEVEL can also be obtained from getprotobyname. OPTVAL might either be a packed string or an integer. An integer OPTVAL is shorthand for pack("i", OPTVAL).

An example disabling Nagle's algorithm on a socket:

use Socket qw(IPPROTO_TCP TCP_NODELAY);
setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);

Portability issues: "setsockopt" in perlport.

shift ARRAY

shift

Shifts the first value of the array off and returns it, shortening the array by 1 and moving everything down. If there are no elements in the array, returns the undefined value. If ARRAY is omitted, shifts the @_ array within the lexical scope of subroutines and formats, and the @ARGV array outside a subroutine and also within the lexical scopes established by the eval STRING, BEGIN {}, INIT {}, CHECK {}, UNITCHECK {}, and END {} constructs.

Starting with Perl 5.14, an experimental feature allowed shift to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.

See also unshift, push, and pop. shift and unshift do the same thing to the left end of an array that pop and push do to the right end.

shmctl ID,CMD,ARG

Calls the System V IPC function shmctl. You'll probably have to say

use IPC::SysV;

first to get the correct constant definitions. If CMD is IPC_STAT, then ARG must be a variable that will hold the returned shmid_ds structure. Returns like ioctl: undef for error; "0 but true" for zero; and the actual return value otherwise. See also "SysV IPC" in perlipc and the documentation for IPC::SysV.

Portability issues: "shmctl" in perlport.

shmget KEY,SIZE,FLAGS

Calls the System V IPC function shmget. Returns the shared memory segment id, or undef on error. See also "SysV IPC" in perlipc and the documentation for IPC::SysV.

Portability issues: "shmget" in perlport.

shmread ID,VAR,POS,SIZE

shmwrite ID,STRING,POS,SIZE

Reads or writes the System V shared memory segment ID starting at position POS for size SIZE by attaching to it, copying in/out, and detaching from it. When reading, VAR must be a variable that will hold the data read. When writing, if STRING is too long, only SIZE bytes are used; if STRING is too short, nulls are written to fill out SIZE bytes. Return true if successful, false on error. shmread taints the variable. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and the IPC::Shareable module from CPAN.

Portability issues: "shmread" in perlport and "shmwrite" in perlport.

shutdown SOCKET,HOW

Shuts down a socket connection in the manner indicated by HOW, which has the same interpretation as in the syscall of the same name.

shutdown($socket, 0);    # I/we have stopped reading data
shutdown($socket, 1);    # I/we have stopped writing data
shutdown($socket, 2);    # I/we have stopped using this socket

This is useful with sockets when you want to tell the other side you're done writing but not done reading, or vice versa. It's also a more insistent form of close because it also disables the file descriptor in any forked copies in other processes.

Returns 1 for success; on error, returns undef if the first argument is not a valid filehandle, or returns 0 and sets $! for any other failure.

sin EXPR

sin

Returns the sine of EXPR (expressed in radians). If EXPR is omitted, returns sine of $_.

For the inverse sine operation, you may use the Math::Trig::asin function, or use this relation:

sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }

sleep EXPR

sleep

Causes the script to sleep for (integer) EXPR seconds, or forever if no argument is given. Returns the integer number of seconds actually slept.

May be interrupted if the process receives a signal such as SIGALRM.

eval {
    local $SIG{ALRM} = sub { die "Alarm!\n" };
    sleep;
};
die $@ unless $@ eq "Alarm!\n";

You probably cannot mix alarm and sleep calls, because sleep is often implemented using alarm.

On some older systems, it may sleep up to a full second less than what you requested, depending on how it counts seconds. Most modern systems always sleep the full amount. They may appear to sleep longer than that, however, because your process might not be scheduled right away in a busy multitasking system.

For delays of finer granularity than one second, the Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) provides usleep. You may also use Perl's four-argument version of select leaving the first three arguments undefined, or you might be able to use the syscall interface to access setitimer(2) if your system supports it. See perlfaq8 for details.

See also the POSIX module's pause function.

socket SOCKET,DOMAIN,TYPE,PROTOCOL

Opens a socket of the specified kind and attaches it to filehandle SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the syscall of the same name. You should use Socket first to get the proper definitions imported. See the examples in "Sockets: Client/Server Communication" in perlipc.

On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See "$^F" in perlvar.

socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL

Creates an unnamed pair of sockets in the specified domain, of the specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as for the syscall of the same name. If unimplemented, raises an exception. Returns true if successful.

On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptors, as determined by the value of $^F. See "$^F" in perlvar.

Some systems define pipe in terms of socketpair, in which a call to pipe($rdr, $wtr) is essentially:

use Socket;
socketpair(my $rdr, my $wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown($rdr, 1);        # no more writing for reader
shutdown($wtr, 0);        # no more reading for writer

See perlipc for an example of socketpair use. Perl 5.8 and later will emulate socketpair using IP sockets to localhost if your system implements sockets but not socketpair.

Portability issues: "socketpair" in perlport.

sort SUBNAME LIST

sort BLOCK LIST

sort LIST

In list context, this sorts the LIST and returns the sorted list value. In scalar context, the behaviour of sort is undefined.

If SUBNAME or BLOCK is omitted, sorts in standard string comparison order. If SUBNAME is specified, it gives the name of a subroutine that returns an integer less than, equal to, or greater than 0, depending on how the elements of the list are to be ordered. (The <=> and cmp operators are extremely useful in such routines.) SUBNAME may be a scalar variable name (unsubscripted), in which case the value provides the name of (or a reference to) the actual subroutine to use. In place of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort subroutine.

If the subroutine's prototype is ($$), the elements to be compared are passed by reference in @_, as for a normal subroutine. This is slower than unprototyped subroutines, where the elements to be compared are passed into the subroutine as the package global variables $a and $b (see example below). Note that in the latter case, it is usually highly counter-productive to declare $a and $b as lexicals.

If the subroutine is an XSUB, the elements to be compared are pushed on to the stack, the way arguments are usually passed to XSUBs. $a and $b are not set.

The values to be compared are always passed by reference and should not be modified.

You also cannot exit out of the sort block or subroutine using any of the loop control operators described in perlsyn or with goto.

When use locale (but not use locale ':not_characters') is in effect, sort LIST sorts LIST according to the current collation locale. See perllocale.

sort returns aliases into the original list, much as a for loop's index variable aliases the list elements. That is, modifying an element of a list returned by sort (for example, in a foreach, map or grep) actually modifies the element in the original list. This is usually something to be avoided when writing clear code.

Perl 5.6 and earlier used a quicksort algorithm to implement sort. That algorithm was not stable and could go quadratic. (A stable sort preserves the input order of elements that compare equal. Although quicksort's run time is O(NlogN) when averaged over all arrays of length N, the time can be O(N**2), quadratic behavior, for some inputs.) In 5.7, the quicksort implementation was replaced with a stable mergesort algorithm whose worst-case behavior is O(NlogN). But benchmarks indicated that for some inputs, on some platforms, the original quicksort was faster. 5.8 has a sort pragma for limited control of the sort. Its rather blunt control of the underlying algorithm may not persist into future Perls, but the ability to characterize the input or output in implementation independent ways quite probably will.

Examples:

# sort lexically
my @articles = sort @files;

# same thing, but with explicit sort routine
my @articles = sort {$a cmp $b} @files;

# now case-insensitively
my @articles = sort {fc($a) cmp fc($b)} @files;

# same thing in reversed order
my @articles = sort {$b cmp $a} @files;

# sort numerically ascending
my @articles = sort {$a <=> $b} @files;

# sort numerically descending
my @articles = sort {$b <=> $a} @files;

# this sorts the %age hash by value instead of key
# using an in-line function
my @eldest = sort { $age{$b} <=> $age{$a} } keys %age;

# sort using explicit subroutine name
sub byage {
    $age{$a} <=> $age{$b};  # presuming numeric
}
my @sortedclass = sort byage @class;

sub backwards { $b cmp $a }
my @harry  = qw(dog cat x Cain Abel);
my @george = qw(gone chased yz Punished Axed);
print sort @harry;
    # prints AbelCaincatdogx
print sort backwards @harry;
    # prints xdogcatCainAbel
print sort @george, 'to', @harry;
    # prints AbelAxedCainPunishedcatchaseddoggonetoxyz

# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise

my @new = sort {
    ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
                        ||
                fc($a)  cmp  fc($b)
} @old;

# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
my (@nums, @caps);
for (@old) {
    push @nums, ( /=(\d+)/ ? $1 : undef );
    push @caps, fc($_);
}

my @new = @old[ sort {
                       $nums[$b] <=> $nums[$a]
                                ||
                       $caps[$a] cmp $caps[$b]
                     } 0..$#old
              ];

# same thing, but without any temps
my @new = map { $_->[0] }
       sort { $b->[1] <=> $a->[1]
                       ||
              $a->[2] cmp $b->[2]
       } map { [$_, /=(\d+)/, fc($_)] } @old;

# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
package Other;
sub backwards ($$) { $_[1] cmp $_[0]; }  # $a and $b are
                                         # not set here
package main;
my @new = sort Other::backwards @old;

# guarantee stability, regardless of algorithm
use sort 'stable';
my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;

# force use of mergesort (not portable outside Perl 5.8)
use sort '_mergesort';  # note discouraging _
my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;

Warning: syntactical care is required when sorting the list returned from a function. If you want to sort the list returned by the function call find_records(@key), you can use:

my @contact = sort { $a cmp $b } find_records @key;
my @contact = sort +find_records(@key);
my @contact = sort &find_records(@key);
my @contact = sort(find_records(@key));

If instead you want to sort the array @key with the comparison routine find_records() then you can use:

my @contact = sort { find_records() } @key;
my @contact = sort find_records(@key);
my @contact = sort(find_records @key);
my @contact = sort(find_records (@key));

You must not declare $a and $b as lexicals. They are package globals. That means that if you're in the main package and type

my @articles = sort {$b <=> $a} @files;

then $a and $b are $main::a and $main::b (or $::a and $::b), but if you're in the FooPack package, it's the same as typing

my @articles = sort {$FooPack::b <=> $FooPack::a} @files;

The comparison function is required to behave. If it returns inconsistent results (sometimes saying $x[1] is less than $x[2] and sometimes saying the opposite, for example) the results are not well-defined.

Because <=> returns undef when either operand is NaN (not-a-number), be careful when sorting with a comparison function like $a <=> $b any lists that might contain a NaN. The following example takes advantage that NaN != NaN to eliminate any NaNs from the input list.

my @result = sort { $a <=> $b } grep { $_ == $_ } @input;

splice ARRAY,OFFSET,LENGTH,LIST

splice ARRAY,OFFSET,LENGTH

splice ARRAY,OFFSET

splice ARRAY

Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with the elements of LIST, if any. In list context, returns the elements removed from the array. In scalar context, returns the last element removed, or undef if no elements are removed. The array grows or shrinks as necessary. If OFFSET is negative then it starts that far from the end of the array. If LENGTH is omitted, removes everything from OFFSET onward. If LENGTH is negative, removes the elements from OFFSET onward except for -LENGTH elements at the end of the array. If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is past the end of the array and a LENGTH was provided, Perl issues a warning, and splices at the end of the array.

The following equivalences hold (assuming $#a >= $i )

push(@a,$x,$y)      splice(@a,@a,0,$x,$y)
pop(@a)             splice(@a,-1)
shift(@a)           splice(@a,0,1)
unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)
$a[$i] = $y         splice(@a,$i,1,$y)

splice can be used, for example, to implement n-ary queue processing:

sub nary_print {
  my $n = shift;
  while (my @next_n = splice @_, 0, $n) {
    say join q{ -- }, @next_n;
  }
}

nary_print(3, qw(a b c d e f g h));
# prints:
#   a -- b -- c
#   d -- e -- f
#   g -- h

Starting with Perl 5.14, an experimental feature allowed splice to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.

split /PATTERN/,EXPR,LIMIT

split /PATTERN/,EXPR

split /PATTERN/

split

Splits the string EXPR into a list of strings and returns the list in list context, or the size of the list in scalar context.

If only PATTERN is given, EXPR defaults to $_.

Anything in EXPR that matches PATTERN is taken to be a separator that separates the EXPR into substrings (called "fields") that do not include the separator. Note that a separator may be longer than one character or even have no characters at all (the empty string, which is a zero-width match).

The PATTERN need not be constant; an expression may be used to specify a pattern that varies at runtime.

If PATTERN matches the empty string, the EXPR is split at the match position (between characters). As an example, the following:

print join(':', split(/b/, 'abc')), "\n";

uses the b in 'abc' as a separator to produce the output a:c. However, this:

print join(':', split(//, 'abc')), "\n";

uses empty string matches as separators to produce the output a:b:c; thus, the empty string may be used to split EXPR into a list of its component characters.

As a special case for split, the empty pattern given in match operator syntax (//) specifically matches the empty string, which is contrary to its usual interpretation as the last successful match.

If PATTERN is /^/, then it is treated as if it used the multiline modifier (/^/m), since it isn't much use otherwise.

As another special case, split emulates the default behavior of the command line tool awk when the PATTERN is either omitted or a string composed of a single space character (such as ' ' or "\x20", but not e.g. / /). In this case, any leading whitespace in EXPR is removed before splitting occurs, and the PATTERN is instead treated as if it were /\s+/; in particular, this means that any contiguous whitespace (not just a single space character) is used as a separator. However, this special treatment can be avoided by specifying the pattern / / instead of the string " ", thereby allowing only a single space character to be a separator. In earlier Perls this special case was restricted to the use of a plain " " as the pattern argument to split; in Perl 5.18.0 and later this special case is triggered by any expression which evaluates to the simple string " ".

If omitted, PATTERN defaults to a single space, " ", triggering the previously described awk emulation.

If LIMIT is specified and positive, it represents the maximum number of fields into which the EXPR may be split; in other words, LIMIT is one greater than the maximum number of times EXPR may be split. Thus, the LIMIT value 1 means that EXPR may be split a maximum of zero times, producing a maximum of one field (namely, the entire value of EXPR). For instance:

print join(':', split(//, 'abc', 1)), "\n";

produces the output abc, and this:

print join(':', split(//, 'abc', 2)), "\n";

produces the output a:bc, and each of these:

print join(':', split(//, 'abc', 3)), "\n";
print join(':', split(//, 'abc', 4)), "\n";

produces the output a:b:c.

If LIMIT is negative, it is treated as if it were instead arbitrarily large; as many fields as possible are produced.

If LIMIT is omitted (or, equivalently, zero), then it is usually treated as if it were instead negative but with the exception that trailing empty fields are stripped (empty leading fields are always preserved); if all fields are empty, then all fields are considered to be trailing (and are thus stripped in this case). Thus, the following:

print join(':', split(/,/, 'a,b,c,,,')), "\n";

produces the output a:b:c, but the following:

print join(':', split(/,/, 'a,b,c,,,', -1)), "\n";

produces the output a:b:c:::.

In time-critical applications, it is worthwhile to avoid splitting into more fields than necessary. Thus, when assigning to a list, if LIMIT is omitted (or zero), then LIMIT is treated as though it were one larger than the number of variables in the list; for the following, LIMIT is implicitly 3:

my ($login, $passwd) = split(/:/);

Note that splitting an EXPR that evaluates to the empty string always produces zero fields, regardless of the LIMIT specified.

An empty leading field is produced when there is a positive-width match at the beginning of EXPR. For instance:

print join(':', split(/ /, ' abc')), "\n";

produces the output :abc. However, a zero-width match at the beginning of EXPR never produces an empty field, so that:

print join(':', split(//, ' abc'));

produces the output :a:b:c (rather than : :a:b:c).

An empty trailing field, on the other hand, is produced when there is a match at the end of EXPR, regardless of the length of the match (of course, unless a non-zero LIMIT is given explicitly, such fields are removed, as in the last example). Thus:

print join(':', split(//, ' abc', -1)), "\n";

produces the output :a:b:c:.

If the PATTERN contains capturing groups, then for each separator, an additional field is produced for each substring captured by a group (in the order in which the groups are specified, as per backreferences); if any group does not match, then it captures the undef value instead of a substring. Also, note that any such additional field is produced whenever there is a separator (that is, whenever a split occurs), and such an additional field does not count towards the LIMIT. Consider the following expressions evaluated in list context (each returned list is provided in the associated comment):

split(/-|,/, "1-10,20", 3)
# ('1', '10', '20')

split(/(-|,)/, "1-10,20", 3)
# ('1', '-', '10', ',', '20')

split(/-|(,)/, "1-10,20", 3)
# ('1', undef, '10', ',', '20')

split(/(-)|,/, "1-10,20", 3)
# ('1', '-', '10', undef, '20')

split(/(-)|(,)/, "1-10,20", 3)
# ('1', '-', undef, '10', undef, ',', '20')

sprintf FORMAT, LIST

Returns a string formatted by the usual printf conventions of the C library function sprintf. See below for more details and see sprintf(3) or printf(3) on your system for an explanation of the general principles.

For example:

# Format number with up to 8 leading zeroes
my $result = sprintf("%08d", $number);

# Round number to 3 digits after decimal point
my $rounded = sprintf("%.3f", $number);

Perl does its own sprintf formatting: it emulates the C function sprintf(3), but doesn't use it except for floating-point numbers, and even then only standard modifiers are allowed. Non-standard extensions in your local sprintf(3) are therefore unavailable from Perl.

Unlike printf, sprintf does not do what you probably mean when you pass it an array as your first argument. The array is given scalar context, and instead of using the 0th element of the array as the format, Perl will use the count of elements in the array as the format, which is almost never useful.

Perl's sprintf permits the following universally-known conversions:

%%    a percent sign
%c    a character with the given number
%s    a string
%d    a signed integer, in decimal
%u    an unsigned integer, in decimal
%o    an unsigned integer, in octal
%x    an unsigned integer, in hexadecimal
%e    a floating-point number, in scientific notation
%f    a floating-point number, in fixed decimal notation
%g    a floating-point number, in %e or %f notation

In addition, Perl permits the following widely-supported conversions:

%X    like %x, but using upper-case letters
%E    like %e, but using an upper-case "E"
%G    like %g, but with an upper-case "E" (if applicable)
%b    an unsigned integer, in binary
%B    like %b, but using an upper-case "B" with the # flag
%p    a pointer (outputs the Perl value's address in hexadecimal)
%n    special: *stores* the number of characters output so far
      into the next argument in the parameter list
%a    hexadecimal floating point
%A    like %a, but using upper-case letters

Finally, for backward (and we do mean "backward") compatibility, Perl permits these unnecessary but widely-supported conversions:

%i    a synonym for %d
%D    a synonym for %ld
%U    a synonym for %lu
%O    a synonym for %lo
%F    a synonym for %f

Note that the number of exponent digits in the scientific notation produced by %e, %E, %g and %G for numbers with the modulus of the exponent less than 100 is system-dependent: it may be three or less (zero-padded as necessary). In other words, 1.23 times ten to the 99th may be either "1.23e99" or "1.23e099". Similarly for %a and %A: the exponent or the hexadecimal digits may float: especially the "long doubles" Perl configuration option may cause surprises.

Between the % and the format letter, you may specify several additional attributes controlling the interpretation of the format. In order, these are:

format parameter index

An explicit format parameter index, such as 2$. By default sprintf will format the next unused argument in the list, but this allows you to take the arguments out of order:

printf '%2$d %1$d', 12, 34;      # prints "34 12"
printf '%3$d %d %1$d', 1, 2, 3;  # prints "3 1 1"

flags

one or more of:

space   prefix non-negative number with a space
+       prefix non-negative number with a plus sign
-       left-justify within the field
0       use zeros, not spaces, to right-justify
#       ensure the leading "0" for any octal,
        prefix non-zero hexadecimal with "0x" or "0X",
        prefix non-zero binary with "0b" or "0B"

For example:

printf '<% d>',  12;   # prints "< 12>"
printf '<% d>',   0;   # prints "< 0>"
printf '<% d>', -12;   # prints "<-12>"
printf '<%+d>',  12;   # prints "<+12>"
printf '<%+d>',   0;   # prints "<+0>"
printf '<%+d>', -12;   # prints "<-12>"
printf '<%6s>',  12;   # prints "<    12>"
printf '<%-6s>', 12;   # prints "<12    >"
printf '<%06s>', 12;   # prints "<000012>"
printf '<%#o>',  12;   # prints "<014>"
printf '<%#x>',  12;   # prints "<0xc>"
printf '<%#X>',  12;   # prints "<0XC>"
printf '<%#b>',  12;   # prints "<0b1100>"
printf '<%#B>',  12;   # prints "<0B1100>"

When a space and a plus sign are given as the flags at once, the space is ignored.

printf '<%+ d>', 12;   # prints "<+12>"
printf '<% +d>', 12;   # prints "<+12>"

When the # flag and a precision are given in the %o conversion, the precision is incremented if it's necessary for the leading "0".

printf '<%#.5o>', 012;      # prints "<00012>"
printf '<%#.5o>', 012345;   # prints "<012345>"
printf '<%#.0o>', 0;        # prints "<0>"

vector flag

This flag tells Perl to interpret the supplied string as a vector of integers, one for each character in the string. Perl applies the format to each integer in turn, then joins the resulting strings with a separator (a dot . by default). This can be useful for displaying ordinal values of characters in arbitrary strings:

printf "%vd", "AB\x{100}";           # prints "65.66.256"
printf "version is v%vd\n", $^V;     # Perl's version

Put an asterisk * before the v to override the string to use to separate the numbers:

printf "address is %*vX\n", ":", $addr;   # IPv6 address
printf "bits are %0*v8b\n", " ", $bits;   # random bitstring

You can also explicitly specify the argument number to use for the join string using something like *2$v; for example:

printf '%*4$vX %*4$vX %*4$vX',       # 3 IPv6 addresses
        @addr[1..3], ":";

(minimum) width

Arguments are usually formatted to be only as wide as required to display the given value. You can override the width by putting a number here, or get the width from the next argument (with *) or from a specified argument (e.g., with *2$):

printf "<%s>", "a";       # prints "<a>"
printf "<%6s>", "a";      # prints "<     a>"
printf "<%*s>", 6, "a";   # prints "<     a>"
printf '<%*2$s>', "a", 6; # prints "<     a>"
printf "<%2s>", "long";   # prints "<long>" (does not truncate)

If a field width obtained through * is negative, it has the same effect as the - flag: left-justification.

precision, or maximum width

You can specify a precision (for numeric conversions) or a maximum width (for string conversions) by specifying a . followed by a number. For floating-point formats except g and G, this specifies how many places right of the decimal point to show (the default being 6). For example:

# these examples are subject to system-specific variation
printf '<%f>', 1;    # prints "<1.000000>"
printf '<%.1f>', 1;  # prints "<1.0>"
printf '<%.0f>', 1;  # prints "<1>"
printf '<%e>', 10;   # prints "<1.000000e+01>"
printf '<%.1e>', 10; # prints "<1.0e+01>"

For "g" and "G", this specifies the maximum number of digits to show, including those prior to the decimal point and those after it; for example:

# These examples are subject to system-specific variation.
printf '<%g>', 1;        # prints "<1>"
printf '<%.10g>', 1;     # prints "<1>"
printf '<%g>', 100;      # prints "<100>"
printf '<%.1g>', 100;    # prints "<1e+02>"
printf '<%.2g>', 100.01; # prints "<1e+02>"
printf '<%.5g>', 100.01; # prints "<100.01>"
printf '<%.4g>', 100.01; # prints "<100>"

For integer conversions, specifying a precision implies that the output of the number itself should be zero-padded to this width, where the 0 flag is ignored:

printf '<%.6d>', 1;      # prints "<000001>"
printf '<%+.6d>', 1;     # prints "<+000001>"
printf '<%-10.6d>', 1;   # prints "<000001    >"
printf '<%10.6d>', 1;    # prints "<    000001>"
printf '<%010.6d>', 1;   # prints "<    000001>"
printf '<%+10.6d>', 1;   # prints "<   +000001>"

printf '<%.6x>', 1;      # prints "<000001>"
printf '<%#.6x>', 1;     # prints "<0x000001>"
printf '<%-10.6x>', 1;   # prints "<000001    >"
printf '<%10.6x>', 1;    # prints "<    000001>"
printf '<%010.6x>', 1;   # prints "<    000001>"
printf '<%#10.6x>', 1;   # prints "<  0x000001>"

For string conversions, specifying a precision truncates the string to fit the specified width:

printf '<%.5s>', "truncated";   # prints "<trunc>"
printf '<%10.5s>', "truncated"; # prints "<     trunc>"

You can also get the precision from the next argument using .*, or from a specified argument (e.g., with .*2$):

printf '<%.6x>', 1;       # prints "<000001>"
printf '<%.*x>', 6, 1;    # prints "<000001>"

printf '<%.*2$x>', 1, 6;  # prints "<000001>"

printf '<%6.*2$x>', 1, 4; # prints "<  0001>"

If a precision obtained through * is negative, it counts as having no precision at all.

printf '<%.*s>',  7, "string";   # prints "<string>"
printf '<%.*s>',  3, "string";   # prints "<str>"
printf '<%.*s>',  0, "string";   # prints "<>"
printf '<%.*s>', -1, "string";   # prints "<string>"

printf '<%.*d>',  1, 0;   # prints "<0>"
printf '<%.*d>',  0, 0;   # prints "<>"
printf '<%.*d>', -1, 0;   # prints "<0>"

size

For numeric conversions, you can specify the size to interpret the number as using l, h, V, q, L, or ll. For integer conversions (d u o x X b i D U O), numbers are usually assumed to be whatever the default integer size is on your platform (usually 32 or 64 bits), but you can override this to use instead one of the standard C types, as supported by the compiler used to build Perl:

hh          interpret integer as C type "char" or "unsigned
            char" on Perl 5.14 or later
h           interpret integer as C type "short" or
            "unsigned short"
j           interpret integer as C type "intmax_t" on Perl
            5.14 or later, and only with a C99 compiler
            (unportable)
l           interpret integer as C type "long" or
            "unsigned long"
q, L, or ll interpret integer as C type "long long",
            "unsigned long long", or "quad" (typically
            64-bit integers)
t           interpret integer as C type "ptrdiff_t" on Perl
            5.14 or later
z           interpret integer as C type "size_t" on Perl 5.14
            or later

As of 5.14, none of these raises an exception if they are not supported on your platform. However, if warnings are enabled, a warning of the printf warning class is issued on an unsupported conversion flag. Should you instead prefer an exception, do this:

use warnings FATAL => "printf";

If you would like to know about a version dependency before you start running the program, put something like this at its top:

use 5.014;  # for hh/j/t/z/ printf modifiers

You can find out whether your Perl supports quads via Config:

use Config;
if ($Config{use64bitint} eq "define"
    || $Config{longsize} >= 8) {
    print "Nice quads!\n";
}

For floating-point conversions (e f g E F G), numbers are usually assumed to be the default floating-point size on your platform (double or long double), but you can force "long double" with q, L, or ll if your platform supports them. You can find out whether your Perl supports long doubles via Config:

use Config;
print "long doubles\n" if $Config{d_longdbl} eq "define";

You can find out whether Perl considers "long double" to be the default floating-point size to use on your platform via Config:

use Config;
if ($Config{uselongdouble} eq "define") {
    print "long doubles by default\n";
}

It can also be that long doubles and doubles are the same thing:

use Config;
($Config{doublesize} == $Config{longdblsize}) &&
        print "doubles are long doubles\n";

The size specifier V has no effect for Perl code, but is supported for compatibility with XS code. It means "use the standard size for a Perl integer or floating-point number", which is the default.

order of arguments

Normally, sprintf takes the next unused argument as the value to format for each format specification. If the format specification uses * to require additional arguments, these are consumed from the argument list in the order they appear in the format specification before the value to format. Where an argument is specified by an explicit index, this does not affect the normal order for the arguments, even when the explicitly specified index would have been the next argument.

So:

printf "<%*.*s>", $a, $b, $c;

uses $a for the width, $b for the precision, and $c as the value to format; while:

printf '<%*1$.*s>', $a, $b;

would use $a for the width and precision, and $b as the value to format.

Here are some more examples; be aware that when using an explicit index, the $ may need escaping:

printf "%2\$d %d\n",      12, 34;     # will print "34 12\n"
printf "%2\$d %d %d\n",   12, 34;     # will print "34 12 34\n"
printf "%3\$d %d %d\n",   12, 34, 56; # will print "56 12 34\n"
printf "%2\$*3\$d %d\n",  12, 34,  3; # will print " 34 12\n"
printf "%*1\$.*f\n",       4,  5, 10; # will print "5.0000\n"

If use locale (including use locale ':not_characters') is in effect and POSIX::setlocale has been called, the character used for the decimal separator in formatted floating-point numbers is affected by the LC_NUMERIC locale. See perllocale and POSIX.

sqrt EXPR

sqrt

Return the positive square root of EXPR. If EXPR is omitted, uses $_. Works only for non-negative operands unless you've loaded the Math::Complex module.

use Math::Complex;
print sqrt(-4);    # prints 2i

srand EXPR

srand

Sets and returns the random number seed for the rand operator.

The point of the function is to "seed" the rand function so that rand can produce a different sequence each time you run your program. When called with a parameter, srand uses that for the seed; otherwise it (semi-)randomly chooses a seed. In either case, starting with Perl 5.14, it returns the seed. To signal that your code will work only on Perls of a recent vintage:

use 5.014;	# so srand returns the seed

If srand is not called explicitly, it is called implicitly without a parameter at the first use of the rand operator. However, there are a few situations where programs are likely to want to call srand. One is for generating predictable results, generally for testing or debugging. There, you use srand($seed), with the same $seed each time. Another case is that you may want to call srand after a fork to avoid child processes sharing the same seed value as the parent (and consequently each other).

Do not call srand() (i.e., without an argument) more than once per process. The internal state of the random number generator should contain more entropy than can be provided by any seed, so calling srand again actually loses randomness.

Most implementations of srand take an integer and will silently truncate decimal numbers. This means srand(42) will usually produce the same results as srand(42.1). To be safe, always pass srand an integer.

A typical use of the returned seed is for a test program which has too many combinations to test comprehensively in the time available to it each run. It can test a random subset each time, and should there be a failure, log the seed used for that run so that it can later be used to reproduce the same results.

stat FILEHANDLE

stat EXPR

stat DIRHANDLE

stat

Returns a 13-element list giving the status info for a file, either the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is omitted, it stats $_ (not _!). Returns the empty list if stat fails. Typically used as follows:

my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
    $atime,$mtime,$ctime,$blksize,$blocks)
       = stat($filename);

Not all fields are supported on all filesystem types. Here are the meanings of the fields:

 0 dev      device number of filesystem
 1 ino      inode number
 2 mode     file mode  (type and permissions)
 3 nlink    number of (hard) links to the file
 4 uid      numeric user ID of file's owner
 5 gid      numeric group ID of file's owner
 6 rdev     the device identifier (special files only)
 7 size     total size of file, in bytes
 8 atime    last access time in seconds since the epoch
 9 mtime    last modify time in seconds since the epoch
10 ctime    inode change time in seconds since the epoch (*)
11 blksize  preferred I/O size in bytes for interacting with the
            file (may vary from file to file)
12 blocks   actual number of system-specific blocks allocated
            on disk (often, but not always, 512 bytes each)

(The epoch was at 00:00 January 1, 1970 GMT.)

(*) Not all fields are supported on all filesystem types. Notably, the ctime field is non-portable. In particular, you cannot expect it to be a "creation time"; see "Files and Filesystems" in perlport for details.

If stat is passed the special filehandle consisting of an underline, no stat is done, but the current contents of the stat structure from the last stat, lstat, or filetest are returned. Example:

if (-x $file && (($d) = stat(_)) && $d < 0) {
    print "$file is executable NFS file\n";
}

(This works on machines only for which the device number is negative under NFS.)

Because the mode contains both the file type and its permissions, you should mask off the file type portion and (s)printf using a "%o" if you want to see the real permissions.

my $mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;

In scalar context, stat returns a boolean value indicating success or failure, and, if successful, sets the information associated with the special filehandle _.

The File::stat module provides a convenient, by-name access mechanism:

use File::stat;
my $sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
       $filename, $sb->size, $sb->mode & 07777,
       scalar localtime $sb->mtime;

You can import symbolic mode constants (S_IF*) and functions (S_IS*) from the Fcntl module:

use Fcntl ':mode';

my $mode = (stat($filename))[2];

my $user_rwx      = ($mode & S_IRWXU) >> 6;
my $group_read    = ($mode & S_IRGRP) >> 3;
my $other_execute =  $mode & S_IXOTH;

printf "Permissions are %04o\n", S_IMODE($mode), "\n";

my $is_setuid     =  $mode & S_ISUID;
my $is_directory  =  S_ISDIR($mode);

You could write the last two using the -u and -d operators. Commonly available S_IF* constants are:

# Permissions: read, write, execute, for user, group, others.

S_IRWXU S_IRUSR S_IWUSR S_IXUSR
S_IRWXG S_IRGRP S_IWGRP S_IXGRP
S_IRWXO S_IROTH S_IWOTH S_IXOTH

# Setuid/Setgid/Stickiness/SaveText.
# Note that the exact meaning of these is system-dependent.

S_ISUID S_ISGID S_ISVTX S_ISTXT

# File types.  Not all are necessarily available on
# your system.

S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
S_IFIFO S_IFSOCK S_IFWHT S_ENFMT

# The following are compatibility aliases for S_IRUSR,
# S_IWUSR, and S_IXUSR.

S_IREAD S_IWRITE S_IEXEC

and the S_IF* functions are

S_IMODE($mode)    the part of $mode containing the permission
                  bits and the setuid/setgid/sticky bits

S_IFMT($mode)     the part of $mode containing the file type
                  which can be bit-anded with (for example)
                  S_IFREG or with the following functions

# The operators -f, -d, -l, -b, -c, -p, and -S.

S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)

# No direct -X operator counterpart, but for the first one
# the -g operator is often equivalent.  The ENFMT stands for
# record flocking enforcement, a platform-dependent feature.

S_ISENFMT($mode) S_ISWHT($mode)

See your native chmod(2) and stat(2) documentation for more details about the S_* constants. To get status info for a symbolic link instead of the target file behind the link, use the lstat function.

Portability issues: "stat" in perlport.

state VARLIST

state TYPE VARLIST

state VARLIST : ATTRS

state TYPE VARLIST : ATTRS

state declares a lexically scoped variable, just like my. However, those variables will never be reinitialized, contrary to lexical variables that are reinitialized each time their enclosing block is entered. See "Persistent Private Variables" in perlsub for details.

If more than one variable is listed, the list must be placed in parentheses. With a parenthesised list, undef can be used as a dummy placeholder. However, since initialization of state variables in list context is currently not possible this would serve no purpose.

study SCALAR

study

Note that since Perl version 5.16 this function has been a no-op, but this might change in a future release.

May take extra time to study SCALAR ($_ if unspecified) in anticipation of doing many pattern matches on the string before it is next modified. This may or may not save time, depending on the nature and number of patterns you are searching and the distribution of character frequencies in the string to be searched; you probably want to compare run times with and without it to see which is faster. Those loops that scan for many short constant strings (including the constant parts of more complex patterns) will benefit most.

(The way study used to work is this: a linked list of every character in the string to be searched is made, so we know, for example, where all the 'k' characters are. From each search string, the rarest character is selected, based on some static frequency tables constructed from some C programs and English text. Only those places that contain this "rarest" character are examined.)

For example, here is a loop that inserts index producing entries before any line containing a certain pattern:

while (<>) {
    study;
    print ".IX foo\n"    if /\bfoo\b/;
    print ".IX bar\n"    if /\bbar\b/;
    print ".IX blurfl\n" if /\bblurfl\b/;
    # ...
    print;
}

In searching for /\bfoo\b/, only locations in $_ that contain f will be looked at, because f is rarer than o. In general, this is a big win except in pathological cases. The only question is whether it saves you more time than it took to build the linked list in the first place.

Note that if you have to look for strings that you don't know till runtime, you can build an entire loop as a string and eval that to avoid recompiling all your patterns all the time. Together with undefining $/ to input entire files as one record, this can be quite fast, often faster than specialized programs like fgrep(1). The following scans a list of files (@files) for a list of words (@words), and prints out the names of those files that contain a match:

my $search = 'local $/; while (<>) { study;';
foreach my $word (@words) {
    $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
my %seen;
eval $search;        # this screams
foreach my $file (sort keys(%seen)) {
    print $file, "\n";
}

sub NAME BLOCK

sub NAME (PROTO) BLOCK

sub NAME : ATTRS BLOCK

sub NAME (PROTO) : ATTRS BLOCK

This is subroutine definition, not a real function per se. Without a BLOCK it's just a forward declaration. Without a NAME, it's an anonymous function declaration, so does return a value: the CODE ref of the closure just created.

See perlsub and perlref for details about subroutines and references; see attributes and Attribute::Handlers for more information about attributes.

__SUB__

A special token that returns a reference to the current subroutine, or undef outside of a subroutine.

The behaviour of __SUB__ within a regex code block (such as /(?{...})/) is subject to change.

This token is only available under use v5.16 or the "current_sub" feature. See feature.

substr EXPR,OFFSET,LENGTH,REPLACEMENT

substr EXPR,OFFSET,LENGTH

substr EXPR,OFFSET

Extracts a substring out of EXPR and returns it. First character is at offset zero. If OFFSET is negative, starts that far back from the end of the string. If LENGTH is omitted, returns everything through the end of the string. If LENGTH is negative, leaves that many characters off the end of the string.

my $s = "The black cat climbed the green tree";
my $color  = substr $s, 4, 5;      # black
my $middle = substr $s, 4, -11;    # black cat climbed the
my $end    = substr $s, 14;        # climbed the green tree
my $tail   = substr $s, -4;        # tree
my $z      = substr $s, -4, 2;     # tr

You can use the substr function as an lvalue, in which case EXPR must itself be an lvalue. If you assign something shorter than LENGTH, the string will shrink, and if you assign something longer than LENGTH, the string will grow to accommodate it. To keep the string the same length, you may need to pad or chop your value using sprintf.

If OFFSET and LENGTH specify a substring that is partly outside the string, only the part within the string is returned. If the substring is beyond either end of the string, substr returns the undefined value and produces a warning. When used as an lvalue, specifying a substring that is entirely outside the string raises an exception. Here's an example showing the behavior for boundary cases:

my $name = 'fred';
substr($name, 4) = 'dy';         # $name is now 'freddy'
my $null = substr $name, 6, 2;   # returns "" (no warning)
my $oops = substr $name, 7;      # returns undef, with warning
substr($name, 7) = 'gap';        # raises an exception

An alternative to using substr as an lvalue is to specify the replacement string as the 4th argument. This allows you to replace parts of the EXPR and return what was there before in one operation, just as you can with splice.

my $s = "The black cat climbed the green tree";
my $z = substr $s, 14, 7, "jumped from";    # climbed
# $s is now "The black cat jumped from the green tree"

Note that the lvalue returned by the three-argument version of substr acts as a 'magic bullet'; each time it is assigned to, it remembers which part of the original string is being modified; for example:

my $x = '1234';
for (substr($x,1,2)) {
    $_ = 'a';   print $x,"\n";    # prints 1a4
    $_ = 'xyz'; print $x,"\n";    # prints 1xyz4
    $x = '56789';
    $_ = 'pq';  print $x,"\n";    # prints 5pq9
}

With negative offsets, it remembers its position from the end of the string when the target string is modified:

my $x = '1234';
for (substr($x, -3, 2)) {
    $_ = 'a';   print $x,"\n";    # prints 1a4, as above
    $x = 'abcdefg';
    print $_,"\n";                # prints f
}

Prior to Perl version 5.10, the result of using an lvalue multiple times was unspecified. Prior to 5.16, the result with negative offsets was unspecified.

symlink OLDFILE,NEWFILE

Creates a new filename symbolically linked to the old filename. Returns 1 for success, 0 otherwise. On systems that don't support symbolic links, raises an exception. To check for that, use eval:

my $symlink_exists = eval { symlink("",""); 1 };

Portability issues: "symlink" in perlport.

syscall NUMBER, LIST

Calls the system call specified as the first element of the list, passing the remaining elements as arguments to the system call. If unimplemented, raises an exception. The arguments are interpreted as follows: if a given argument is numeric, the argument is passed as an int. If not, the pointer to the string value is passed. You are responsible to make sure a string is pre-extended long enough to receive any result that might be written into a string. You can't use a string literal (or other read-only string) as an argument to syscall because Perl has to assume that any string pointer might be written through. If your integer arguments are not literals and have never been interpreted in a numeric context, you may need to add 0 to them to force them to look like numbers. This emulates the syswrite function (or vice versa):

require 'syscall.ph';        # may need to run h2ph
my $s = "hi there\n";
syscall(SYS_write(), fileno(STDOUT), $s, length $s);

Note that Perl supports passing of up to only 14 arguments to your syscall, which in practice should (usually) suffice.

Syscall returns whatever value returned by the system call it calls. If the system call fails, syscall returns -1 and sets $! (errno). Note that some system calls can legitimately return -1. The proper way to handle such calls is to assign $! = 0 before the call, then check the value of $! if syscall returns -1.

There's a problem with syscall(SYS_pipe()): it returns the file number of the read end of the pipe it creates, but there is no way to retrieve the file number of the other end. You can avoid this problem by using pipe instead.

Portability issues: "syscall" in perlport.

sysopen FILEHANDLE,FILENAME,MODE

sysopen FILEHANDLE,FILENAME,MODE,PERMS

Opens the file whose filename is given by FILENAME, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real filehandle wanted; an undefined scalar will be suitably autovivified. This function calls the underlying operating system's open(2) function with the parameters FILENAME, MODE, and PERMS.

Returns true on success and undef otherwise.

The possible values and flag bits of the MODE parameter are system-dependent; they are available via the standard module Fcntl. See the documentation of your operating system's open(2) syscall to see which values and flag bits are available. You may combine several flags using the |-operator.

Some of the most common values are O_RDONLY for opening the file in read-only mode, O_WRONLY for opening the file in write-only mode, and O_RDWR for opening the file in read-write mode.

For historical reasons, some values work on almost every system supported by Perl: 0 means read-only, 1 means write-only, and 2 means read/write. We know that these values do not work under OS/390 and on the Macintosh; you probably don't want to use them in new code.

If the file named by FILENAME does not exist and the open call creates it (typically because MODE includes the O_CREAT flag), then the value of PERMS specifies the permissions of the newly created file. If you omit the PERMS argument to sysopen, Perl uses the octal value 0666. These permission values need to be in octal, and are modified by your process's current umask.

In many systems the O_EXCL flag is available for opening files in exclusive mode. This is not locking: exclusiveness means here that if the file already exists, sysopen fails. O_EXCL may not work on network filesystems, and has no effect unless the O_CREAT flag is set as well. Setting O_CREAT|O_EXCL prevents the file from being opened if it is a symbolic link. It does not protect against symbolic links in the file's path.

Sometimes you may want to truncate an already-existing file. This can be done using the O_TRUNC flag. The behavior of O_TRUNC with O_RDONLY is undefined.

You should seldom if ever use 0644 as argument to sysopen, because that takes away the user's option to have a more permissive umask. Better to omit it. See umask for more on this.

Note that under Perls older than 5.8.0, sysopen depends on the fdopen(3) C library function. On many Unix systems, fdopen(3) is known to fail when file descriptors exceed a certain value, typically 255. If you need more file descriptors than that, consider using the POSIX::open function. For Perls 5.8.0 and later, PerlIO is (most often) the default.

See perlopentut for a kinder, gentler explanation of opening files.

Portability issues: "sysopen" in perlport.

sysread FILEHANDLE,SCALAR,LENGTH,OFFSET

sysread FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE, using read(2). It bypasses buffered IO, so mixing this with other kinds of reads, print, write, seek, tell, or eof can cause confusion because the perlio or stdio layers usually buffer data. Returns the number of bytes actually read, 0 at end of file, or undef if there was an error (in the latter case $! is also set). SCALAR will be grown or shrunk so that the last byte actually read is the last byte of the scalar after the read.

There is no syseof() function, which is ok, since eof doesn't work well on device files (like ttys) anyway. Use sysread and check for a return value for 0 to decide whether you're done.

Note that if the filehandle has been marked as :utf8, Unicode characters are read instead of bytes (the LENGTH, OFFSET, and the return value of sysread are in Unicode characters). The :encoding(...) layer implicitly introduces the :utf8 layer. See binmode, open, and the open pragma.

sysseek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are 0 to set the new position to POSITION; 1 to set the it to the current position plus POSITION; and 2 to set it to EOF plus POSITION, typically negative.

Note the in bytes: even if the filehandle has been set to operate on characters (for example by using the :encoding(utf8) I/O layer), tell will return byte offsets, not character offsets (because implementing that would render sysseek unacceptably slow).

sysseek bypasses normal buffered IO, so mixing it with reads other than sysread (for example readline or read), print, write, seek, tell, or eof may cause confusion.

For WHENCE, you may also use the constants SEEK_SET, SEEK_CUR, and SEEK_END (start of the file, current position, end of the file) from the Fcntl module. Use of the constants is also more portable than relying on 0, 1, and 2. For example to define a "systell" function:

use Fcntl 'SEEK_CUR';
sub systell { sysseek($_[0], 0, SEEK_CUR) }

Returns the new position, or the undefined value on failure. A position of zero is returned as the string "0 but true"; thus sysseek returns true on success and false on failure, yet you can still easily determine the new position.

system LIST

system PROGRAM LIST

Does exactly the same thing as exec, except that a fork is done first and the parent process waits for the child process to exit. Note that argument processing varies depending on the number of arguments. If there is more than one argument in LIST, or if LIST is an array with more than one value, starts the program given by the first element of the list with arguments given by the rest of the list. If there is only one scalar argument, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is /bin/sh -c on Unix platforms, but varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly to execvp, which is more efficient. On Windows, only the system PROGRAM LIST syntax will reliably avoid using the shell; system LIST, even with more than one element, will fall back to the shell if the first spawn fails.

The return value is the exit status of the program as returned by the wait call. To get the actual exit value, shift right by eight (see below). See also exec. This is not what you want to use to capture the output from a command; for that you should use merely backticks or qx//, as described in "`STRING`" in perlop. Return value of -1 indicates a failure to start the program or an error of the wait(2) system call (inspect $! for the reason).

If you'd like to make system (and many other bits of Perl) die on error, have a look at the autodie pragma.

Like exec, system allows you to lie to a program about its name if you use the system PROGRAM LIST syntax. Again, see exec.

Since SIGINT and SIGQUIT are ignored during the execution of system, if you expect your program to terminate on receipt of these signals you will need to arrange to do so yourself based on the return value.

my @args = ("command", "arg1", "arg2");
system(@args) == 0
    or die "system @args failed: $?";

If you'd like to manually inspect system's failure, you can check all possible failure modes by inspecting $? like this:

if ($? == -1) {
    print "failed to execute: $!\n";
}
elsif ($? & 127) {
    printf "child died with signal %d, %s coredump\n",
        ($? & 127),  ($? & 128) ? 'with' : 'without';
}
else {
    printf "child exited with value %d\n", $? >> 8;
}

Alternatively, you may inspect the value of ${^CHILD_ERROR_NATIVE} with the W*() calls from the POSIX module.

When system's arguments are executed indirectly by the shell, results and return codes are subject to its quirks. See "`STRING`" in perlop and exec for details.

Since system does a fork and wait it may affect a SIGCHLD handler. See perlipc for details.

Portability issues: "system" in perlport.

syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET

syswrite FILEHANDLE,SCALAR,LENGTH

syswrite FILEHANDLE,SCALAR

Attempts to write LENGTH bytes of data from variable SCALAR to the specified FILEHANDLE, using write(2). If LENGTH is not specified, writes whole SCALAR. It bypasses buffered IO, so mixing this with reads (other than sysread)), print, write, seek, tell, or eof may cause confusion because the perlio and stdio layers usually buffer data. Returns the number of bytes actually written, or undef if there was an error (in this case the errno variable $! is also set). If the LENGTH is greater than the data available in the SCALAR after the OFFSET, only as much data as is available will be written.

An OFFSET may be specified to write the data from some part of the string other than the beginning. A negative OFFSET specifies writing that many characters counting backwards from the end of the string. If SCALAR is of length zero, you can only use an OFFSET of 0.

WARNING: If the filehandle is marked :utf8, Unicode characters encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and return value of syswrite are in (UTF8-encoded Unicode) characters. The :encoding(...) layer implicitly introduces the :utf8 layer. Alternately, if the handle is not marked with an encoding but you attempt to write characters with code points over 255, raises an exception. See binmode, open, and the open pragma.

tell FILEHANDLE

tell

Returns the current position in bytes for FILEHANDLE, or -1 on error. FILEHANDLE may be an expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted, assumes the file last read.

The return value of tell for the standard streams like the STDIN depends on the operating system: it may return -1 or something else. tell on pipes, fifos, and sockets usually returns -1.

There is no systell function. Use sysseek($fh, 0, 1) for that.

Do not use tell (or other buffered I/O operations) on a filehandle that has been manipulated by sysread, syswrite, or sysseek. Those functions ignore the buffering, while tell does not.

telldir DIRHANDLE

Returns the current position of the readdir routines on DIRHANDLE. Value may be given to seekdir to access a particular location in a directory. telldir has the same caveats about possible directory compaction as the corresponding system library routine.

tie VARIABLE,CLASSNAME,LIST

This function binds a variable to a package class that will provide the implementation for the variable. VARIABLE is the name of the variable to be enchanted. CLASSNAME is the name of a class implementing objects of correct type. Any additional arguments are passed to the appropriate constructor method of the class (meaning TIESCALAR, TIEHANDLE, TIEARRAY, or TIEHASH). Typically these are arguments such as might be passed to the dbm_open(3) function of C. The object returned by the constructor is also returned by the tie function, which would be useful if you want to access other methods in CLASSNAME.

Note that functions such as keys and values may return huge lists when used on large objects, like DBM files. You may prefer to use the each function to iterate over such. Example:

# print out history file offsets
use NDBM_File;
tie(my %HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (my ($key,$val) = each %HIST) {
    print $key, ' = ', unpack('L', $val), "\n";
}

A class implementing a hash should have the following methods:

TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
SCALAR this
DESTROY this
UNTIE this

A class implementing an ordinary array should have the following methods:

TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
FETCHSIZE this
STORESIZE this, count
CLEAR this
PUSH this, LIST
POP this
SHIFT this
UNSHIFT this, LIST
SPLICE this, offset, length, LIST
EXTEND this, count
DELETE this, key
EXISTS this, key
DESTROY this
UNTIE this

A class implementing a filehandle should have the following methods:

TIEHANDLE classname, LIST
READ this, scalar, length, offset
READLINE this
GETC this
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
BINMODE this
EOF this
FILENO this
SEEK this, position, whence
TELL this
OPEN this, mode, LIST
CLOSE this
DESTROY this
UNTIE this

A class implementing a scalar should have the following methods:

TIESCALAR classname, LIST
FETCH this,
STORE this, value
DESTROY this
UNTIE this

Not all methods indicated above need be implemented. See perltie, Tie::Hash, Tie::Array, Tie::Scalar, and Tie::Handle.

Unlike dbmopen, the tie function will not use or require a module for you; you need to do that explicitly yourself. See DB_File or the Config module for interesting tie implementations.

For further details see perltie, tied.

tied VARIABLE

Returns a reference to the object underlying VARIABLE (the same value that was originally returned by the tie call that bound the variable to a package.) Returns the undefined value if VARIABLE isn't tied to a package.

time

Returns the number of non-leap seconds since whatever time the system considers to be the epoch, suitable for feeding to gmtime and localtime. On most systems the epoch is 00:00:00 UTC, January 1, 1970; a prominent exception being Mac OS Classic which uses 00:00:00, January 1, 1904 in the current local time zone for its epoch.

For measuring time in better granularity than one second, use the Time::HiRes module from Perl 5.8 onwards (or from CPAN before then), or, if you have gettimeofday(2), you may be able to use the syscall interface of Perl. See perlfaq8 for details.

For date and time processing look at the many related modules on CPAN. For a comprehensive date and time representation look at the DateTime module.

times

Returns a four-element list giving the user and system times in seconds for this process and any exited children of this process.

my ($user,$system,$cuser,$csystem) = times;

In scalar context, times returns $user.

Children's times are only included for terminated children.

Portability issues: "times" in perlport.

tr///

The transliteration operator. Same as y///. See "Quote-Like Operators" in perlop.

truncate FILEHANDLE,LENGTH

truncate EXPR,LENGTH

Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. Raises an exception if truncate isn't implemented on your system. Returns true if successful, undef on error.

The behavior is undefined if LENGTH is greater than the length of the file.

The position in the file of FILEHANDLE is left unchanged. You may want to call seek before writing to the file.

Portability issues: "truncate" in perlport.

uc EXPR

uc

Returns an uppercased version of EXPR. This is the internal function implementing the \U escape in double-quoted strings. It does not attempt to do titlecase mapping on initial letters. See ucfirst for that.

If EXPR is omitted, uses $_.

This function behaves the same way under various pragmas, such as in a locale, as lc does.

ucfirst EXPR

ucfirst

Returns the value of EXPR with the first character in uppercase (titlecase in Unicode). This is the internal function implementing the \u escape in double-quoted strings.

If EXPR is omitted, uses $_.

This function behaves the same way under various pragmas, such as in a locale, as lc does.

umask EXPR

umask

Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, merely returns the current umask.

The Unix permission rwxr-x--- is represented as three sets of three bits, or three octal digits: 0750 (the leading 0 indicates octal and isn't one of the digits). The umask value is such a number representing disabled permissions bits. The permission (or "mode") values you pass mkdir or sysopen are modified by your umask, so even if you tell sysopen to create a file with permissions 0777, if your umask is 0022, then the file will actually be created with permissions 0755. If your umask were 0027 (group can't write; others can't read, write, or execute), then passing sysopen 0666 would create a file with mode 0640 (because 0666 &~ 027 is 0640).

Here's some advice: supply a creation mode of 0666 for regular files (in sysopen) and one of 0777 for directories (in mkdir) and executable files. This gives users the freedom of choice: if they want protected files, they might choose process umasks of 022, 027, or even the particularly antisocial mask of 077. Programs should rarely if ever make policy decisions better left to the user. The exception to this is when writing files that should be kept private: mail files, web browser cookies, .rhosts files, and so on.

If umask(2) is not implemented on your system and you are trying to restrict access for yourself (i.e., (EXPR & 0700) > 0), raises an exception. If umask(2) is not implemented and you are not trying to restrict access for yourself, returns undef.

Remember that a umask is a number, usually given in octal; it is not a string of octal digits. See also oct, if all you have is a string.

Portability issues: "umask" in perlport.

undef EXPR

undef

Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an array (using @), a hash (using %), a subroutine (using &), or a typeglob (using *). Saying undef $hash{$key} will probably not do what you expect on most predefined variables or DBM list values, so don't do that; see delete. Always returns the undefined value. You can omit the EXPR, in which case nothing is undefined, but you still get an undefined value that you could, for instance, return from a subroutine, assign to a variable, or pass as a parameter. Examples:

undef $foo;
undef $bar{'blurfl'};      # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz;       # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
my ($x, $y, undef, $z) = foo();    # Ignore third value returned

Note that this is a unary operator, not a list operator.

unlink LIST

unlink

Deletes a list of files. On success, it returns the number of files it successfully deleted. On failure, it returns false and sets $! (errno):

my $unlinked = unlink 'a', 'b', 'c';
unlink @goners;
unlink glob "*.bak";

On error, unlink will not tell you which files it could not remove. If you want to know which files you could not remove, try them one at a time:

foreach my $file ( @goners ) {
    unlink $file or warn "Could not unlink $file: $!";
}

Note: unlink will not attempt to delete directories unless you are superuser and the -U flag is supplied to Perl. Even if these conditions are met, be warned that unlinking a directory can inflict damage on your filesystem. Finally, using unlink on directories is not supported on many operating systems. Use rmdir instead.

If LIST is omitted, unlink uses $_.

unpack TEMPLATE,EXPR

unpack TEMPLATE

unpack does the reverse of pack: it takes a string and expands it out into a list of values. (In scalar context, it returns merely the first value produced.)

If EXPR is omitted, unpacks the $_ string. See perlpacktut for an introduction to this function.

The string is broken into chunks described by the TEMPLATE. Each chunk is converted separately to a value. Typically, either the string is a result of pack, or the characters of the string represent a C structure of some kind.

The TEMPLATE has the same format as in the pack function. Here's a subroutine that does substring:

sub substr {
    my ($what, $where, $howmuch) = @_;
    unpack("x$where a$howmuch", $what);
}

and then there's

sub ordinal { unpack("W",$_[0]); } # same as ord()

In addition to fields allowed in pack, you may prefix a field with a %<number> to indicate that you want a <number>-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. The checksum is calculated by summing numeric values of expanded values (for string fields the sum of ord($char) is taken; for bit fields the sum of zeroes and ones).

For example, the following computes the same number as the System V sum program:

my $checksum = do {
    local $/;  # slurp!
    unpack("%32W*", readline) % 65535;
};

The following efficiently counts the number of set bits in a bit vector:

my $setbits = unpack("%32b*", $selectmask);

The p and P formats should be used with care. Since Perl has no way of checking whether the value passed to unpack corresponds to a valid memory location, passing a pointer value that's not known to be valid is likely to have disastrous consequences.

If there are more pack codes or if the repeat count of a field or a group is larger than what the remainder of the input string allows, the result is not well defined: the repeat count may be decreased, or unpack may produce empty strings or zeros, or it may raise an exception. If the input string is longer than one described by the TEMPLATE, the remainder of that input string is ignored.

See pack for more examples and notes.

unshift ARRAY,LIST

Does the opposite of a shift. Or the opposite of a push, depending on how you look at it. Prepends list to the front of the array and returns the new number of elements in the array.

unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;

Note the LIST is prepended whole, not one element at a time, so the prepended elements stay in the same order. Use reverse to do the reverse.

Starting with Perl 5.14, an experimental feature allowed unshift to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.

untie VARIABLE

Breaks the binding between a variable and a package. (See tie.) Has no effect if the variable is not tied.

use Module VERSION LIST

use Module VERSION

use Module LIST

use Module

use VERSION

Imports some semantics into the current package from the named module, generally by aliasing certain subroutine or variable names into your package. It is exactly equivalent to

BEGIN { require Module; Module->import( LIST ); }

except that Module must be a bareword. The importation can be made conditional by using the if module.

In the peculiar use VERSION form, VERSION may be either a positive decimal fraction such as 5.006, which will be compared to $], or a v-string of the form v5.6.1, which will be compared to $^V (aka $PERL_VERSION). An exception is raised if VERSION is greater than the version of the current Perl interpreter; Perl will not attempt to parse the rest of the file. Compare with require, which can do a similar check at run time. Symmetrically, no VERSION allows you to specify that you want a version of Perl older than the specified one.

Specifying VERSION as a literal of the form v5.6.1 should generally be avoided, because it leads to misleading error messages under earlier versions of Perl (that is, prior to 5.6.0) that do not support this syntax. The equivalent numeric version should be used instead.

use v5.6.1;     # compile time version check
use 5.6.1;      # ditto
use 5.006_001;  # ditto; preferred for backwards compatibility

This is often useful if you need to check the current Perl version before useing library modules that won't work with older versions of Perl. (We try not to do this more than we have to.)

use VERSION also lexically enables all features available in the requested version as defined by the feature pragma, disabling any features not in the requested version's feature bundle. See feature. Similarly, if the specified Perl version is greater than or equal to 5.12.0, strictures are enabled lexically as with use strict. Any explicit use of use strict or no strict overrides use VERSION, even if it comes before it. Later use of use VERSION will override all behavior of a previous use VERSION, possibly removing the strict and feature added by use VERSION. use VERSION does not load the feature.pm or strict.pm files.

The BEGIN forces the require and import to happen at compile time. The require makes sure the module is loaded into memory if it hasn't been yet. The import is not a builtin; it's just an ordinary static method call into the Module package to tell the module to import the list of features back into the current package. The module can implement its import method any way it likes, though most modules just choose to derive their import method via inheritance from the Exporter class that is defined in the Exporter module. See Exporter. If no import method can be found, then the call is skipped, even if there is an AUTOLOAD method.

If you do not want to call the package's import method (for instance, to stop your namespace from being altered), explicitly supply the empty list:

use Module ();

That is exactly equivalent to

BEGIN { require Module }

If the VERSION argument is present between Module and LIST, then the use will call the VERSION method in class Module with the given version as an argument:

use Module 12.34;

is equivalent to:

BEGIN { require Module; Module->VERSION(12.34) }

The default VERSION method, inherited from the UNIVERSAL class, croaks if the given version is larger than the value of the variable $Module::VERSION.

Again, there is a distinction between omitting LIST (import called with no arguments) and an explicit empty LIST () (import not called). Note that there is no comma after VERSION!

Because this is a wide-open interface, pragmas (compiler directives) are also implemented this way. Some of the currently implemented pragmas are:

use constant;
use diagnostics;
use integer;
use sigtrap  qw(SEGV BUS);
use strict   qw(subs vars refs);
use subs     qw(afunc blurfl);
use warnings qw(all);
use sort     qw(stable _quicksort _mergesort);

Some of these pseudo-modules import semantics into the current block scope (like strict or integer, unlike ordinary modules, which import symbols into the current package (which are effective through the end of the file).

Because use takes effect at compile time, it doesn't respect the ordinary flow control of the code being compiled. In particular, putting a use inside the false branch of a conditional doesn't prevent it from being processed. If a module or pragma only needs to be loaded conditionally, this can be done using the if pragma:

use if $] < 5.008, "utf8";
use if WANT_WARNINGS, warnings => qw(all);

There's a corresponding no declaration that unimports meanings imported by use, i.e., it calls Module->unimport(LIST) instead of import. It behaves just as import does with VERSION, an omitted or empty LIST, or no unimport method being found.

no integer;
no strict 'refs';
no warnings;

Care should be taken when using the no VERSION form of no. It is only meant to be used to assert that the running Perl is of a earlier version than its argument and not to undo the feature-enabling side effects of use VERSION.

See perlmodlib for a list of standard modules and pragmas. See perlrun for the -M and -m command-line options to Perl that give use functionality from the command-line.

utime LIST

Changes the access and modification times on each file of a list of files. The first two elements of the list must be the NUMERIC access and modification times, in that order. Returns the number of files successfully changed. The inode change time of each file is set to the current time. For example, this code has the same effect as the Unix touch(1) command when the files already exist and belong to the user running the program:

#!/usr/bin/perl
my $atime = my $mtime = time;
utime $atime, $mtime, @ARGV;

Since Perl 5.8.0, if the first two elements of the list are undef, the utime(2) syscall from your C library is called with a null second argument. On most systems, this will set the file's access and modification times to the current time (i.e., equivalent to the example above) and will work even on files you don't own provided you have write permission:

    for my $file (@ARGV) {
	utime(undef, undef, $file)
	    || warn "Couldn't touch $file: $!";
    }

Under NFS this will use the time of the NFS server, not the time of the local machine. If there is a time synchronization problem, the NFS server and local machine will have different times. The Unix touch(1) command will in fact normally use this form instead of the one shown in the first example.

Passing only one of the first two elements as undef is equivalent to passing a 0 and will not have the effect described when both are undef. This also triggers an uninitialized warning.

On systems that support futimes(2), you may pass filehandles among the files. On systems that don't support futimes(2), passing filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized; barewords are considered filenames.

Portability issues: "utime" in perlport.

values HASH

values ARRAY

In list context, returns a list consisting of all the values of the named hash. In Perl 5.12 or later only, will also return a list of the values of an array; prior to that release, attempting to use an array argument will produce a syntax error. In scalar context, returns the number of values.

As a side effect, calling values resets the HASH or ARRAY's internal iterator, see each. (In particular, calling values in void context resets the iterator with no other overhead. Apart from resetting the iterator, values @array in list context is the same as plain @array. (We recommend that you use void context keys @array for this, but reasoned that taking values @array out would require more documentation than leaving it in.)

Note that the values are not copied, which means modifying them will modify the contents of the hash:

for (values %hash)      { s/foo/bar/g }  # modifies %hash values
for (@hash{keys %hash}) { s/foo/bar/g }  # same

Starting with Perl 5.14, an experimental feature allowed values to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.

use 5.012;	# so keys/values/each work on arrays

Non-function Keywords by Cross-reference

perldata

__DATA__
__END__: These keywords are documented in "Special Literals" in perldata.

perlmod

BEGIN
CHECK
END
INIT
UNITCHECK: These compile phase keywords are documented in "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod.

perlobj

DESTROY: This method keyword is documented in "Destructors" in perlobj.

perlop

and
cmp
eq
ge
gt
le
lt
ne
not
or
x
xor: These operators are documented in perlop.

perlsub

AUTOLOAD: This keyword is documented in "Autoloading" in perlsub.

perlsyn

else

elsif

for

foreach

if

unless

until

while

These flow-control keywords are documented in "Compound Statements" in perlsyn.

elseif

The "else if" keyword is spelled elsif in Perl. There's no elif or else if either. It does parse elseif, but only to warn you about not using it.

See the documentation for flow-control keywords in "Compound Statements" in perlsyn.

default
given
when: These flow-control keywords related to the experimental switch feature are documented in "Switch Statements" in perlsyn.

To install vars, copy and paste the appropriate command in to your terminal.

cpanm

cpanm vars

CPAN shell

perl -MCPAN -e shell
install vars

For more information on module installation, please visit the detailed CPAN module installation guide.

	Global
`s`	Focus search bar
`?`	Bring up this help dialog

	GitHub
`g` `p`	Go to pull requests
`g` `i`	go to github issues (only if github is preferred repository)

	POD
`g` `a`	Go to author
`g` `c`	Go to changes
`g` `i`	Go to issues
`g` `d`	Go to dist
`g` `r`	Go to repository/SCM
`g` `s`	Go to source
`g` `b`	Go to file browse

	Search terms
module: (e.g. module:Plugin)
distribution: (e.g. distribution:Dancer auth)
author: (e.g. author:SONGMU Redis)
version: (e.g. version:1.00)

NAME

DESCRIPTION

Perl Functions by Category

Portability

Alphabetical Listing of Perl Functions

Non-function Keywords by Cross-reference

perldata

perlmod

perlobj

perlop

perlsub

perlsyn

Module Install Instructions