I recently had to do something incredibly similar to this but I made use of HTML::TreeBuilder. My code ran down into something alot shorter (I've re-written mine here so its more of a psuedo code than anything):

while(my ($tag, $att_list) = each %TAGS)
{
    my @list = $tree->look_down('_tag',$tag);
    foreach my $item (@list)
    {
        foreach my $attr(@{$att_list})
        {
            next unless $item->attr($attr);
            next unless length (my $a = $item->attr($attr));
            next if $a =~ /mailto:/;
            my $target = URI->new_abs($a, $base);
            
            next unless $self->compareHosts($target,$url);
            next unless $self->validScheme($url,['http']);
            next unless $self->allowed($url);
            
            unless(exists $self->[GOOD_LINKS]->{$a})
            {
                $item->attr($attr,$target->rel($base));
                push @{$queue}, $target;
            }
        }
    }
}
[download]

I pasted this, out of interest really, as we've used two different approaches to do roughly the same thing. I was hoping some sort of comparison on technique could be made (etc). However, be aware that mine turns all absolute URLS to relative (or attempts to) as I'm mucking about with the resource later on in my app.

Cheers

Simon

Update:

Naturally, $self and $tree refer to objects and methods that I haven't mentioned in my snippet. $tree is the TreeBuilder instance for those who couldn't guess.