/ 
Perl-Critic-Policy-Variables-ProhibitUselessVarClearing-0.001
/ Perl::Critic::Policy::Variables::ProhibitUselessVarClearing

NAME

Perl::Critic::Policy::Variables::ProhibitUselessVarClearing - Do not empty a lexical that nothing reads again before it goes out of scope.

VERSION

version 0.001

Perl::Critic::Policy::Variables::ProhibitUselessVarClearing

A lexical goes away at the end of the block that declares it. Perl frees it then, and nothing that comes after the block can reach it. So emptying one that nothing reads again is a statement that does nothing:

if ($wanted) {
    my %secrets = lookup(%wanted);
    apply( $config, %secrets );
    %secrets = ();                  # violates: the block ends, and so does %secrets
}

This is a habit from C, where memory stays until something frees it. In Perl, the end of the scope frees it, as perldoc perlguts says under garbage collection. Clearing it by hand costs a statement, and a reader has to look for the later read that would explain it, which does not exist.

PROHIBITED

A statement that only empties a my variable, when the rest of the block that declares the variable never reads it again:

%h = ();
@a = ();
$x = undef;
$hr = {};
$ar = [];
undef %h;
undef @a;
undef $x;
undef($x);

ALLOWED

%h = ();  f(%h);                    # read again later
my %h;  $r = \%h;  %h = ();         # a reference can still reach it
my @a;  my $f = sub { @a };  @a = ();   # so can a closure
my %h;  for (@x) { ...; %h = () }   # the next pass reads it
our %h;  %h = ();                   # not a lexical
state %h;  %h = ();                 # lives across calls
my %h;  sub f { %h = () }           # the next call reads it

A mention of the variable inside a string, a regular expression or a heredoc counts as a read, whether or not it interpolates.

WHAT IT CANNOT SEE

Emptying what a reference points to is not reported:

%$hr = ();
@{$ar} = ();
undef %$hr;

That empties a hash or an array that other code can also hold a reference to, and nothing in the source says whether it does. So the statement can matter even when nothing here reads $hr again. Giving the scalar a new, empty reference, as $hr = {} does, changes only the scalar, and is reported.

WHEN THE CLEARING DOES SOMETHING

Emptying a variable runs the DESTROY of what it held at that moment, rather than at the end of the block. Releasing a lock or closing a handle before the slow part of a block is a reason to clear a variable early. Say so where it happens:

undef $lock;    ## no critic (ProhibitUselessVarClearing) -- release the lock before the upload

CONFIGURATION

None.

METHODS

What Perl::Critic::Policy asks of a policy, answered here rather than called from anywhere.

supported_parameters

None.

default_severity

Low: the statement does nothing, and costs only a reader's time.

default_themes

maintenance.

applies_to

A plain statement, which is what an assignment or a call to undef is.

violates

Reports a statement that only empties a lexical, when nothing can read the lexical again before the end of the block that declares it.

BUGS

Please report any bugs or feature requests on the bugtracker website https://github.com/teodesian/perl-critic-policy-prohibituselessvarclearing/issues

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.

AUTHORS

Current Maintainers:

  • George S. Baugh <george@troglodyne.net>

COPYRIGHT AND LICENSE

Copyright (c) 2026 Troglodyne LLC

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.