/ 
Devel-Monitor-0.9.0.7
River stage zero No dependents
/ Devel::Monitor

NAME

Devel::Monitor - Monitor your variables/objects for memory leaks

DESCRIPTION

You have memory leaks, and you want to remove it... You can use this tool to help you find which variables/objects that are not destroyed when they should be, and thereafter, you can visualise exactly where is the circular reference for some specific variables/objects.

WHAT IT CAN'T DO

Even if your modules are memory leak free, it doesn't mean that external modules that you are using don't have it. So, before running your application on mod_perl, you should be sure that EVERY modules are ok. (In particular those perl extensions calling C++ code)

SYNOPSIS

   use Devel::Monitor qw(:all);

   #-----------------------------------------------------------------------------
   # Monitor scalars, arrays, hashes, references, constants                      
   #-----------------------------------------------------------------------------
   my ($a,$b) = (Foo::Bar->new(), Foo::Bar->new());
   my ($c, @d, %e);
   use constant F => [1,2];
   monitor('name for a' => \$a,
           'name for b' => \$b,
           'name for c' => \$c,
           'name for d' => \@d,
           'name for e' => \%e,
           'name for F' => \&F); #NOTE : Dont add parentheses to the end of the constant (\&F())

   #-----------------------------------------------------------------------------
   # Print circular references                                                   
   #-----------------------------------------------------------------------------
   # NOTE : You cannot use print_circular_ref on a monitored/tied variable 
   #        (See "We cannot use tied objects references because it reuse memory space" doc)
   print_circular_ref(\$a);
   print_circular_ref(\$b);
   print_circular_ref(\$c);
   print_circular_ref(\@d);
   print_circular_ref(\%e);
   print_circular_ref(\&F); #NOTE : Dont add parentheses to the end of the constant (\&F())

USAGE : monitor

Example with a circular reference

+----------------------+
| Code                 |
+----------------------+
{
    my @a;
    monitor('a' => \@a);
    $a[0] = \@a; #Add a circular reference
    print STDERR "Leaving scope\n";
}
print STDERR "Scope left\n";
 
+----------------------+
| Output               |
+----------------------+
MONITOR ARRAY a
Leaving scope
Scope left                       
DESTROY ARRAY a
 
+----------------------+
| Meaning              |
+----------------------+
The line "DESTROY ARRAY a" should be between scope prints.
@a were deleted on program exit.

Example without a circular reference

   +----------------------+
   | Code                 |
   +----------------------+
   {
       my @a;
       monitor('a' => \@a);
       print STDERR "Leaving scope\n";
   }
   print STDERR "Scope left\n";
    
   +----------------------+
   | Output               |
   +----------------------+
   MONITOR ARRAY a
   Leaving scope
   DESTROY ARRAY a
   Scope left
    
   +----------------------+
   | Meaning              |
   +----------------------+
   Everything is ok

Now that you know there is a circular reference, you can track it down using the print_circular_ref method

USAGE : print_circular_ref

Example

   +----------------------+
   | Code                 |
   |         a            |
   |        / \           |
   |      [0] [1]         |
   |      /     \         |
   |  'asdf'     b <--|   |
   |              \   |   |
   |              [3]-|   |
   |                      |
   +----------------------+
   my (@a, @b);
   $a[0] = 'asdf';
   $a[1] = \@b;
   $b[3] = \@b;
   print_circular_ref(\@a);
   print_circular_ref(\@b);

   +----------------------+
   | Output               |
   +----------------------+
   -------------------------------------------------------------------------------
   Checking circular references for ARRAY(0x814e358)
   -------------------------------------------------------------------------------
   Internal circular reference found : ARRAY(0x814e358)[1][3] on ARRAY(0x814e370)
   1 - Item     : ARRAY(0x814e358)
   2 - Source   : [1]
       Item     : ARRAY(0x814e370)
   3 - Source   : [3]
       Item     : ARRAY(0x814e370)
   -------------------------------------------------------------------------------
   -------------------------------------------------------------------------------
   Results for ARRAY(0x814e358)
   Circular reference          : 0
   Internal circular reference : 1
   Weak circular reference     : 0
   -------------------------------------------------------------------------------
   -------------------------------------------------------------------------------
   Checking circular references for ARRAY(0x814e370)
   -------------------------------------------------------------------------------
   Circular reference found : ARRAY(0x814e370)[3]
   1 - Item     : ARRAY(0x814e370)
   2 - Source   : [3]
       Item     : ARRAY(0x814e370)
   -------------------------------------------------------------------------------
   -------------------------------------------------------------------------------
   Results for ARRAY(0x814e370)
   Circular reference          : 1
   Internal circular reference : 0
   Weak circular reference     : 0
   -------------------------------------------------------------------------------

TRACKING MEMORY LEAKS

How to remove Circular references in Perl

#------------------------------------------------------------------------------+
#
# Let's say we have this basic code :
#
#------------------------------------------------------------------------------+
 
#!/usr/bin/perl
 
#--------------------------------------------------------------------
# Little program
#--------------------------------------------------------------------
 
use strict;
use warnings;
use Devel::Monitor qw(:all);
 
{
    my $a = ClassA->new();
    my $b = $a->getClassB();
    monitor('$b' => \$b);
    $b->getClassA()->printSomething();
    print "Leaving scope\n";
}
print "Scope left\n";
 
#--------------------------------------------------------------------
# ClassA (Just a class with the "printSomething" method)
#--------------------------------------------------------------------
 
package ClassA;
use strict;
use warnings;
use Scalar::Util qw(weaken isweak);
 
sub new {
    my ($class) = @_;
    my $self = {};
    bless($self => $class);
    return $self;
}
 
sub getClassB {
    my $self = shift;
    $self->{_classB} = ClassB->new($self);
    return $self->{_classB};
}
 
sub printSomething {
    print "Something\n";
}
 
#--------------------------------------------------------------------
# ClassB (A class that got a "parent" which is a ClassA instance)
#--------------------------------------------------------------------
 
package ClassB;
use strict;
use warnings;
use Scalar::Util qw(weaken isweak);
 
sub new {
    my ($class, $classA) = @_;
    my $self = {};
    bless($self => $class);
    $self->setClassA($classA);
    return $self;
}
 
sub setClassA {
    my ($self, $classA) = @_;
    $self->{_classA} = $classA;
}
 
sub getClassA {
    return shift->{_classA};
}
 
1;
 
#------------------------------------------------------------------------------+
#
# The output will be
#
#------------------------------------------------------------------------------+
 
MONITOR HASH : $b
Something
Leaving scope
Scope left
DESTROY HASH : $b
 
#------------------------------------------------------------------------------+
#
# We see that the object reference by $b isn't destroyed when leaving the scope
# because $a->{_classB} still use it. So, we got a circular reference here. We must
# weaken one side of the circular reference to help Perl disallocate memory.
#
#------------------------------------------------------------------------------+
#------------------------------------------------------------------------------+
# Wrong way to break circular references
#------------------------------------------------------------------------------+
sub getClassB {
    my $self = shift;
    $self->{_classB} = ClassB->new($self);  #$self->{_classB} is the only
                                            #reference to the objects
    weaken($self->{_classB});               #we weaken the only reference,
                                            #so, $self->{_classB} is DESTROYED HERE,
                                            #which is very bad
    print "\$self->{_classB} is now weaken\n" if isweak($self->{_classB});
    return $self->{_classB};
}
#------------------------------------------------------------------------------+
# Good way
#------------------------------------------------------------------------------+
sub getClassB {
    my $self = shift;
    my $b = ClassB->new($self);
    $self->{_classB} = $b;                  #we create a second reference to the object
    weaken($self->{_classB});               #we weaken this reference, which is not deleted
                                            #because thre is another reference
    print "\$self->{_classB} is now weaken\n" if isweak($self->{_classB});
    return $self->{_classB};
}
#------------------------------------------------------------------------------+
# Be careful ! With this code, it won't work
#------------------------------------------------------------------------------+
sub getClassB {
    my $self = shift;
    {
        my $b = ClassB->new($self);
        $self->{_classB} = $b;                  #we create a second reference to the object
        weaken($self->{_classB});               #we weaken this reference, which is not deleted
                                                #because thre is another reference
        print "\$self->{_classB} is now weaken\n" if isweak($self->{_classB});
    } #$b is destroyed here, and the other reference $self->{_classB} is a weak reference,
      #so the ClassB instance is destroyed, $self->{_classB} now equal undef
    return $self->{_classB};
}
#------------------------------------------------------------------------------+
# Good way
#------------------------------------------------------------------------------+
sub getClassB {
    my $self = shift;
    my $b;
    {
        $b = ClassB->new($self);
        $self->{_classB} = $b;                  #we create a second reference to the object
        weaken($self->{_classB});               #we weaken this reference, which is not deleted
                                                #because thre is another reference
        print "\$self->{_classB} is now weaken\n" if isweak($self->{_classB});
    } #$b is still not destroyed, so we didn't lose our not weak reference
    return $self->{_classB}; #We return the object, someone on the other side will now keep
                             #the reference, so we don't care if $b lose the reference.
                             #Our job is done !
}
#------------------------------------------------------------------------------+
#
# Conclusion : You must be sure that you keep a non weak reference to the object
#
#------------------------------------------------------------------------------+
 
#------------------------------------------------------------------------------+
#
# The output (Using the good way) will be
#
#------------------------------------------------------------------------------+
 
$self->{_classB} is now weaken
MONITOR HASH : $b
Something
Leaving scope
DESTROY HASH : $b
Scope left
 
#------------------------------------------------------------------------------+
#
# There is no circular references now...
#
#------------------------------------------------------------------------------+
 
#------------------------------------------------------------------------------+
#
# IMPORTANT : Always weaken the caller's reference because someone may use the
# child objects (ClassB) this way. Let's see what can happen if you don't.
#
# If we get the following code
#
#------------------------------------------------------------------------------+
my $b;
{
    my $a = ClassA->new();
    monitor('$a' => \$a);
    $b = ClassB->new($a);
    $b->getClassA()->printSomething();
    print "Leaving scope\n";
}
print "Scope left\n";
$b->getClassA()->printSomething();
 
#------------------------------------------------------------------------------+
#
# And the sub setClassA
#
#------------------------------------------------------------------------------+
sub setClassA {
    my ($self, $classA) = @_;
    $self->{_classA} = $classA;
    weaken($self->{_classA});
    print "\$self->{_classA} is now weaken\n" if isweak($self->{_classA});
}
 
#------------------------------------------------------------------------------+
#
# You'll get this error
#
#------------------------------------------------------------------------------+
MONITOR HASH : $a
$self->{_classA} is now weaken
Something
Leaving scope
DESTROY HASH : $a
Scope left
Can't call method "printSomething" on an undefined value at test3.pl line 29.
 
#------------------------------------------------------------------------------+
#
# $a is destroyed when leaving the scope, and the other reference to this variable
# is weaken, so this one is destroyed too. This clearly demonstrate that you must
# weaken the caller's reference.
#
#------------------------------------------------------------------------------+

THINGS YOU SHOULD BE AWARE OF

Loop variables are passed by references

   Let's see in details what output you get when monitoring variables inside a loop. 

   +----------------------+
   | Code                 |
   +----------------------+
   {
       my @list = (1,2,3);
       print STDERR join(", ",@list)."\n";
       for my $item (@list) {
           monitor("item $item" => \$item);
           $item+=1000;
           print "$item\n";
       }
       print STDERR join(", ",@list)."\n";
       print "Leaving scope\n";
   }
   print "Scope left\n";
    
   +------------------------+
   | What you might want    |
   |(Or something like that)|
   +------------------------+
   1, 2, 3
   MONITOR SCALAR : item 1
   1001
   DESTROY SCALAR : item 1
   MONITOR SCALAR : item 2
   1002
   DESTROY SCALAR : item 2
   MONITOR SCALAR : item 3
   1003
   DESTROY SCALAR : item 3
   1, 2, 3
   Leaving scope
   Scope left

   +----------------------+
   | Real Output          |
   +----------------------+
   1, 2, 3
   MONITOR SCALAR : item 1
   1001
   MONITOR SCALAR : item 2
   1002
   MONITOR SCALAR : item 3
   1003
   1001, 1002, 1003
   Leaving scope
   DESTROY SCALAR : item 3
   DESTROY SCALAR : item 2
   DESTROY SCALAR : item 1
   Scope left
    
   +----------------------+
   | Meaning              |
   +----------------------+
   Perl passes variables by reference within for/foreach, so the variables you are using
   are the original ones. (You can print the scalar adresses to be sure)
   The difference is that normaly, Perl passes variables by value.
   So, if you monitor those variables, they won't be destroyed until the initial declaration is.

Variable using constants are destroyed when the constant is destroyed

Let's look at this small example :

+----------------------+
| Code                 |
+----------------------+
#!/usr/bin/perl
use strict;
use warnings;
use Devel::Monitor qw(:all);

use constant CONST => [1,2,3]; 
#monitor('CONST', \&CONST);
print &CONST."\n";
{
    my $item = CONST();
    monitor('item', \$item);
    print $item."\n";
    print "Leaving scope\n";
}
print "Scope left\n";

+------------------------+
| What you might want    |
|(Or something like that)|
+------------------------+
ARRAY(0x81c503c)
MONITOR ARRAY : item
ARRAY(0x1234567)
Leaving scope
DESTROY ARRAY : item
Scope left

+----------------------+
| Real Output          |
+----------------------+
ARRAY(0x81c503c)
MONITOR ARRAY : item
ARRAY(0x81c503c)
Leaving scope
Scope left
DESTROY ARRAY : item

+----------------------+
| Meaning              |
+----------------------+
It looks like your variable is not destroyed ! But in fact, $item is the same
reference that CONST is. So, you are monitoring CONST directly ! If you
absolutely want to monitor this code, you must uncomment the 
"#monitor('CONST', \&CONST);" line in code.

+----------------------+
| Output with monitor  |
| on \&CONST           |
+----------------------+
MONITOR CODE SCALAR : CONST [0]
MONITOR CODE SCALAR : CONST [1]
MONITOR CODE SCALAR : CONST [2]
MONITOR CODE ARRAY : CONST
ARRAY(0x81c4e30)
Array from item is already tied by CONST
ARRAY(0x81c4e30)
Leaving scope
Scope left
DESTROY CODE SCALAR : CONST [0]
DESTROY CODE SCALAR : CONST [1]
DESTROY CODE SCALAR : CONST [2]
DESTROY CODE ARRAY : CONST

+----------------------+
| Meaning              |
+----------------------+
You monitored a constant and you cannot monitor twice a variable, so $item won't
be monitored. This way, you can see that there is no memory leak.

Perl problems

You cannot use references from a tied object because it reuse memory space

Let's see in details what happen when you try to print circular references
with a tied object (An object with a monitor by example !!!)

+----------------------+
| Code                 |
+----------------------+
my $self = {'a' => 1,
            'b' => 2};
monitor('self' => \$self);
print STDERR \($self->{'a'})."\n";
print STDERR \($self->{'b'})."\n";
print STDERR \($self->{'a'}).\($self->{'b'})."\n";
foreach my $key (keys %$self) {
    my $keyRef = \$key;
    my $value = $self->{$key};
    my $valueRef = \($self->{$key});
    print STDERR "KEY:$key, KEY REF:$keyRef, VALUE:$value, VALUE REF:$valueRef\n";
}   

+----------------------+
| Output               |
+----------------------+
MONITOR HASH : self
SCALAR(0x8141384)
SCALAR(0x8141384)
SCALAR(0x8141384)SCALAR(0x81413cc)
KEY:a, KEY REF:SCALAR(0x8141420), VALUE:1, VALUE REF:SCALAR(0x824becc)
KEY:b, KEY REF:SCALAR(0x81413cc), VALUE:2, VALUE REF:SCALAR(0x824becc)
DESTROY HASH : self

+----------------------+
| Code 2               |
+----------------------+
my %self;
#monitor('self' => \$self);
tie %self, 'Devel::Monitor::TestHash';
$self{a} = 1;
$self{b} = 2;
print STDERR \($self{a})."\n";
print STDERR \($self{b})."\n";
print STDERR \($self{a}).\($self{b})."\n";
foreach my $key (keys %self) {
    my $keyRef = \$key;
    my $value = $self{$key};
    my $valueRef = \($self{$key});
    print STDERR "KEY:$key, KEY REF:$keyRef, VALUE:$value, VALUE REF:$valueRef\n";
}  

+----------------------+
| Output 2             |
+----------------------+
SCALAR(0x8141378)
SCALAR(0x8141378)
SCALAR(0x8141378)SCALAR(0x8248fe8)
KEY:a, KEY REF:SCALAR(0x81413cc), VALUE:1, VALUE REF:SCALAR(0x825567c)
KEY:b, KEY REF:SCALAR(0x825564c), VALUE:2, VALUE REF:SCALAR(0x825567c)
Devel::Monitor::TestHash::DESTROY : Devel::Monitor::TestHash=HASH(0x81412e8)

+----------------------+
| Meaning              |
+----------------------+
Hash keys refering 1 and 2 can't be the same reference. But we see the
opposite on these small examples. It seems like tied objects reuse memory space
instead of refering to the original value from the untied object.

You cannot weaken a tied object

This is actually an unhandled reference by Perl (Verified with 5.9.2-). It means that if you monitor (or tie explicitly) an object, any weaken references into this one will simply be ignored.

Proof 01 : Basic test

+----------------------+
| Code                 |
+----------------------+
#!/usr/bin/perl

use Scalar::Util qw(weaken isweak);
my (@a, @b);
tie @a, 'Monitor::TestArray';
tie @b, 'Monitor::TestArray';
$a[0] = \@b;
$b[0] = \@a;
weaken($b[0]);
if (isweak($a[0])) {
   print "\$a[0] is weak\n";
} else {
   print "\$a[0] is not weak\n";
}  
if (isweak($b[0])) {
   print "\$b[0] is weak\n";
} else {
   print "\$b[0] is not weak\n";
}    
package Monitor::TestArray;
use Tie::Array;
use base 'Tie::StdArray';

sub DESTROY { "Monitor::TestArray::DESTROY : $_[0]\n"; }

1; 

+----------------------+
| Wanted output        |
+----------------------+
$a[0] is not weak
$b[0] is weak

+----------------------+
| Real output          |
+----------------------+
$a[0] is not weak
$b[0] is not weak

+----------------------+
| Meaning              |
+----------------------+
We still have this output if we remove one of the "tie" call. But, if we remove those
two "tie", it works and we get the wanted output. So there is a problem.

Proof 02 : mod_perl

+----------------------+
| Code                 |
+----------------------+
+------------+
| test.pl    |
+------------+
#!/usr/bin/perl
use strict;
use warnings;
use Scalar::Util qw(weaken);
use Devel::Monitor qw(:all);
use Util::Junk;

my (@a, $b);
#tie @a, 'Devel::Monitor::TestArray';
$a[0] = \$b;
$b = \@a;
$a[1] = Util::Junk::_20M();
weaken($a[0]);

+------------+
| Util::Junk |
+------------+
package Util::Junk;
use strict;
use warnings;

sub _20M() { 'A 20 megs string here filled with zeros' }

1;

+----------------------+
| wget-test.pl         |
+----------------------+
#!/usr/bin/perl

use strict;
use warnings;

my $baseUrl = 'http://localhost/perl/test.pl';

my $i = 0;
while (1) {
    print "Loop ".++$i."\n";
    
    system('wget "'.$baseUrl.'" -O /dev/null') == 0
        or die "\nwget failed or has been interrupted : $?\n";
}

+----------------------+
| Test 01              |
+----------------------+
Now that we got a program and a caller (and mod_perl on our apache server), we can start the program.

perl wget-test.pl

When @a is not tied (See the commented tie in test.pl), after loading the page like ten times, the
page will be in cache in every apache processes and other loading will be VERY fast. You'll also
notice that memory is stable.

However, if you uncomment the tie call in test.pl, you'll see your memory being filled to death and
every page loaded will be as long as at the beginning

Proof 03 : Final assault

Firstly, we must be sure that the methods Scalar::Util::weaken and Scalar::Util::isweak
doesn't contain bugs. The code for these method follows : 

void
weaken(sv)
   SV *sv
PROTOTYPE: $
CODE:
#ifdef SvWEAKREF
   sv_rvweaken(sv);
#else
   croak("weak references are not implemented in this release of perl");
#endif

void
isweak(sv)
   SV *sv
PROTOTYPE: $
CODE:
#ifdef SvWEAKREF
   ST(0) = boolSV(SvROK(sv) && SvWEAKREF(sv));
   XSRETURN(1);
#else
   croak("weak references are not implemented in this release of perl");
#endif

We easily see that there is absolutely no problems here.

Now let's see what happen if we dump a tied variable by using Devel::Peek.
It should activate the WEAKREF flag if the reference is weak.

Let's see what result we should get :

+----------------------+
| Code                 |
+----------------------+
#!/usr/bin/perl
use strict;
use warnings;
use Devel::Monitor qw(:all);
 
use Scalar::Util qw(weaken);
use Devel::Peek;
{
    my (@a);
    $a[0] = \@a;
    #tie @a, 'TestArray';
    Dump($a[0],1);
    weaken($a[0]);
    Dump($a[0],1);
    print "Leaving scope\n";
}
print "Scope left\n";
 
package TestArray;
use Tie::Array;
use base 'Tie::StdArray';
 
sub DESTROY { print "Monitor::TestArray::DESTROY : $_[0]\n"; }
 
1;

+-------------------------------+
| Output without the "tie" call |
+-------------------------------+
SV = RV(0x81829c0) at 0x814127c
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x814e740
  SV = PVAV(0x81426cc) at 0x814e740
    REFCNT = 2
    FLAGS = (PADBUSY,PADMY)
    IV = 0
    NV = 0
    ARRAY = 0x8148888
    FILL = 0
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)
SV = RV(0x81829c0) at 0x814127c
  REFCNT = 1
  FLAGS = (ROK,WEAKREF,IsUV)
  RV = 0x814e740
  SV = PVAV(0x81426cc) at 0x814e740
    REFCNT = 1
    FLAGS = (PADBUSY,PADMY,RMG)
    IV = 0
    NV = 0
    MAGIC = 0x8266f08
      MG_VIRTUAL = &PL_vtbl_backref
      MG_TYPE = PERL_MAGIC_backref(<)
      MG_FLAGS = 0x02
        REFCOUNTED
      MG_OBJ = 0x81411c8
      SV = PVAV(0x8263704) at 0x81411c8
        REFCNT = 2
        FLAGS = ()
        IV = 0
        NV = 0
        ARRAY = 0x82677e8
        FILL = 0
        MAX = 3
        ARYLEN = 0x0
        FLAGS = (REAL)
    ARRAY = 0x8148888
    FILL = 0
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)
Leaving scope
Scope left

+----------------------+
| Explanations         |
+----------------------+
We actually see the WEAKREF flag that confirms us that the reference is weak.
However, let's see what happen when we uncomment the 11th line (the tie call on @a)

+----------------------------+
| Output with the "tie" call |
+----------------------------+
SV = PVLV(0x817c568) at 0x81413f0
  REFCNT = 1
  FLAGS = (TEMP,GMG,SMG,RMG)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x81505b8
    MG_VIRTUAL = &PL_vtbl_packelem
    MG_TYPE = PERL_MAGIC_tiedelem(p)
    MG_FLAGS = 0x02
      REFCOUNTED
    MG_OBJ = 0x814139c
    SV = RV(0x81829ac) at 0x814139c
      REFCNT = 2
      FLAGS = (ROK)
      RV = 0x8141354
  TYPE = t
  TARGOFF = 0
  TARGLEN = 0
  TARG = 0x81413f0
SV = PVLV(0x817c568) at 0x81413f0
  REFCNT = 1
  FLAGS = (TEMP,GMG,SMG,RMG)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x81505b8
    MG_VIRTUAL = &PL_vtbl_packelem
    MG_TYPE = PERL_MAGIC_tiedelem(p)
    MG_FLAGS = 0x02
      REFCOUNTED
    MG_OBJ = 0x814139c
    SV = RV(0x81829ac) at 0x814139c
      REFCNT = 2
      FLAGS = (ROK)
      RV = 0x8141354
  TYPE = t
  TARGOFF = 0
  TARGLEN = 0
  TARG = 0x81413f0
Leaving scope
Scope left
Monitor::TestArray::DESTROY : TestArray=ARRAY(0x8141354)

+----------------------+
| Explanations         |
+----------------------+
Absolutely nothing has changed before and after. IT IS A PROBLEM ! So, I debugged
the perl source code to verify what happen with a tied variable. The method goes
like this :

/*
=for apidoc sv_rvweaken
 
Weaken a reference: set the C<SvWEAKREF> flag on this RV; give the
referred-to SV C<PERL_MAGIC_backref> magic if it hasn't already; and
push a back-reference to this RV onto the array of backreferences
associated with that magic.
 
=cut
*/
 
SV *
Perl_sv_rvweaken(pTHX_ SV *sv)
{
    SV *tsv;
    if (!SvOK(sv))  /* let undefs pass */
        return sv;
    if (!SvROK(sv))
        Perl_croak(aTHX_ "Can't weaken a nonreference");
    else if (SvWEAKREF(sv)) {
        if (ckWARN(WARN_MISC))
            Perl_warner(aTHX_ packWARN(WARN_MISC), "Reference is already weak");
        return sv;
    }
    tsv = SvRV(sv);
    sv_add_backref(tsv, sv);
    SvWEAKREF_on(sv);
    SvREFCNT_dec(tsv);
    return sv;
}

The problem is at the line "if (!SvOK(sv))". A tied variable enter this condition
and returns itself without any modifications... The reason is that our variables
has those flags FLAGS = (TEMP,GMG,SMG,RMG). The code should be something like
this :

if (!SvOK(sv))
    if (SvMAGIC(sv)) {
        //***************************************
        //Do something here !!!
        //***************************************
    } else {
        return sv;
    }

This bug has been submitted and is unanswered for now. (See http://rt.perl.org/rt3/Ticket/Display.html?id=34524)

Conclusion

It is actually impossible to weaken a tied variable

TRICKS

Checking modules syntax

Since monitored are executed when you check syntax of a module, it will print out 
to stderr some messages with constants and some global variables. So to remove 
those prints, simple grep it by redirecting stderr to stdout and grep it

perl -c MyModule.pm 2>&1 | grep -iv '^(DESTROY|MONITOR|Scalar constant)'

MODULES THAT PRODUCE MEMORY LEAKS

You must destroy them when you don't need anymore those object instances

+----------------------+
| Bio::Graphics::Panel |
+----------------------+
my $panel = Bio::Graphics::Panel->new(%options);
...
$panel->finished(); #Don't forget to call this destructor
  
+----------------------+
| XML::DOM             |
+----------------------+
my $parser  = new XML::DOM::Parser;
my $doc = $parser->parsefile ("file.xml");
...
$doc->dispose(); #Don't forget to call this destructor

NOTE : I suggest that you use XML::LibXML instead

NOTE

This module has been tested with scalars, hashes, arrays, blessed hashes, blessed arrays, tied hashes, tied arrays, tied scalars.

BUGS

None known

AUTHOR

Philippe Cote < philippe.cote@usherbrooke.ca > Génome Québec < http://www.genomequebec.com >

CREDITS

I got the main idea from a module that is not on CPAN. See http://www.infocopter.com/perl/monitored-variables.htm (Monitor.pm)

COPYRIGHT AND LICENSE

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

1 POD Error

The following errors were encountered while parsing the POD:

Around line 1657:

Non-ASCII character seen before =encoding in 'Génome'. Assuming CP1252