I've been playing with some of the Devel::* modules lately. I came arross Devel::Carnivore. Here is a small example I put together: 
use strict;
use warnings;

use Devel::Carnivore;

my $h;
{
    my %h : Watch('%h');

    $h{fname} = "Foo";
    $h = \%h;    
}
$h->{lname} = 'Bar';

unwatch $h;

$h->{email} = 'foo@bar.com';

[download]
and the output: 
$perl carnivore.pl
# variable is now under observation
> %h: "fname" changed from "" to "Foo"  at carnivore.pl line 10
> %h: "lname" changed from "" to "Bar"  at carnivore.pl line 13
# mission completed

[download]
I have several comments: - When we start and stop watching I would like to see the name we assigned to the variable, better yet I'd like to see the real variable name. I would like to be able to control the watching from the outside world. e.g. in .carnivore I would like to say something like script.pl:37:watch %h; so I can run perl -d:Carnivore script.pl and %h will be watched from line 37 of the script.pl file Any idea if I can do this with another module? Maybe with the debugger?

In reply to debugging: tracing changes to hashes by szabgab

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