Here is another way, using XML::Simple, which changes a few of the opponents, like you wanted.

use strict;
use XML::Simple;

my @data = (<DATA>);

my $xml = XMLin((join'', @data));

foreach my $foo (@{$xml->{'foo'}})
{
    foreach my $opponent (@{$foo->{'opponent'}})
    {
        if($opponent eq 'wolf')
        {
            $opponent = 'Heinz Sielmann';
        }
        elsif($opponent eq 'ant')
        {
            $opponent = 'Peter Scott';
        }
        
    }

}

print XMLout($xml, rootname => 'xml');

__DATA__
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xml>
<xml>
  <foo class="life or death">
    <opponent>wolf</opponent>
    <opponent>ant</opponent>
  </foo>
  <foo class="life or death">
    <opponent>pantomime goose</opponent>
    <opponent>Terrance Rattigan</opponent>
  </foo>
</xml>
[download]

I don't usually code a lot of XML, but when I need to, I find that XML::Simple (together with Data::Dumper) often lets me do simple stuff really quickly. Of course, it takes a little tounge-in-cheek for the dereferencing sometimes, see references quick reference for an excellent tutorial on this. :)

---

You have moved into a dark place.
It is pitch black. You are likely to be eaten by a wolf.