Greetings,

Adding the infinitely advisable:

use strict;
use warnings;
[download]

To your sample yields:

Variable "@imgs" will not stay shared at par.pl line 17 (#1)
[download]

Drilling further down:

C:\TEMP>perl -Mdiagnostics par.pl
perl -Mdiagnostics par.pl
Variable "@imgs" will not stay shared at par.pl line 17 (#1)
    (W closure) An inner (nested) named subroutine is referencing a
    lexical variable defined in an outer subroutine.
    
    When the inner subroutine is called, it will probably see the valu
+e of
    the outer subroutine's variable as it was before and during the *f
+irst*
    call to the outer subroutine; in this case, after the first call t
+o the
    outer subroutine is complete, the inner and outer subroutines will
+ no
    longer share a common value for the variable.  In other words, the
    variable will no longer be shared.
    
    Furthermore, if the outer subroutine is anonymous and references a
    lexical variable outside itself, then the outer and inner subrouti
+nes
    will never share the given variable.
    
    This problem can usually be solved by making the inner subroutine
    anonymous, using the sub {} syntax.  When inner anonymous subs tha
+t
    reference variables in outer subroutines are called or referenced,
+ they
    are automatically rebound to the current values of such variables.
[download]

I could not have said it better myself. :)
Hence the working version:

use strict;
use warnings;
use HTML::LinkExtor;
use LWP::UserAgent;
use URI::URL;

sub parsedocument
{
    my ($url) = @_;
        
      my $ua = LWP::UserAgent->new;
    $ua->env_proxy();
    
    # Set up a callback that collect image links
      my @imgs = ();
      my $callback = sub {
      my($tag, %attr) = @_;
      return if $tag ne 'img';  # we only look closer at <img ...>
      push(@imgs, values %attr);
      };

      my $p = HTML::LinkExtor->new($callback);

      # Request document and parse it as it arrives
      my $res = $ua->request(HTTP::Request->new(GET => $url),
                      sub {$p->parse($_[0])});

    # Expand all image URLs to absolute ones
      my $base = $res->base;
      @imgs = map { $_ = url($_, $base)->abs; } @imgs;

      # Print them out
      print join("<br>", @imgs), "<br>";
}

map {parsedocument($_) } @ARGV;
[download]

Note that your sample's original (from the documentation of HTML::Linkextor) works exactly because it occurs in the program's main. When you wrap it in a sub, you get the problem you described, which is wellknown - for instance - to people trying to use mod_perl and Apache::Registry.
Cheers,
alf

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