Using XML::Rules:

use strict;
use warnings;
use XML::Rules;

my $xml = <<XML;
<?xml version='1.0'?>
<root>
  <test>
    <name>file</name>
    <href>file.txt</href>
  </test>
  <test>
    <name>file1</name>
    <href>file1.txt</href>
  </test>
  <notest>
    <name>file1</name>
    <href>file1.txt</href>
  </notest>
</root>
XML

my $parser = XML::Rules->new (
    style => 'filter',
    rules => {
        _default => 'raw',
        href => sub {
            my ($tag, $attr, $context) = @_;
            $attr->{_content} =~ s/\.txt$/.xml/i if $context->[-1] eq 
+'test';
            return $tag => $attr;
        }
    }
);

$parser->filter($xml);
[download]