#!/usr/bin/perl
use warnings;
use strict;

#use lib q{c:/www/lib}; 
#use SW::Debug;

use HTML::TreeBuilder;

my $html = <<HTML;
<a href="page.html">
 <img src="pic.jpg">
</a>
<span>other stuff</span>
<img alt="pic" width="10" height="10" src="pic.jpg">
<span>more</span>
HTML

my $tb = HTML::TreeBuilder->new_from_content($html);
my @images = $tb->look_down(_tag => q{img}) or die qq{look_down for img failed: $!\n};

for my $img (@images){
  $img->replace_with([q{span}, {id => q{fo_big}}, $img]);
}

print $tb->as_HTML;