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>