For what it's worth, I just tried abusing my laptop with this code:

use strict;
use HTML::Element;
sub funny {
    my $counter = 'x0000';
    my $give_id;
    $give_id = sub {
      my $x = $_[0];
      $x->attr('id', $counter++) unless defined $x->attr('id');
      foreach my $c ($x->content_list) {
        $give_id->($c) if ref $c; # ignore text nodes
      }
    };
    $give_id->($_[0]);
    undef $give_id;  ##### Remove for evil effects #####
  }

my  $a = HTML::Element->new('a', href => 'http://www.perl.com/');

print "Start: ", scalar(localtime), "\n";
for my $i (0..1_000_000) { funny($a); }
print "Finish: ", scalar(localtime), "\n";
[download]

With the undef in place, the difference between the "Start" and "Finish" times was 15 seconds. With the undef commented out, the difference between "Start" and "Finish" was only a little over a minute (1:04); however, five minutes after printing the "Finish" line my laptop was still swapping like mad and the shell prompt hadn't yet come back - this indicates that perl was having a devil of a time cleaning things up on exit. (I just gave up and hit Ctrl-C at that point)

Incidentally, while the version without an undef was running, it took a full 30 seconds for my laptop to display the Ctrl-Alt-Del screen after I gave it the three-finger salute.

I'm not going to try the 16_000_000 times I suggested above.