#!/usr/bin/perl -- 

use strict;
use warnings;


use XML::Simple;
use Data::Dumper;

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

print $xml,$/;

my $data = XMLin($xml);

print Dumper( $data );

for my $key( keys %{ $data->{test} } ){
    $data->{test}{$key}{href} =~ s/\.txt$/.xml/;
}

print XMLout($data, RootName=>'root', NoAttr=>1,XMLDecl => 1);

__END__
<?xml version='1.0'?>
<root>
  <test>
    <name>file</name>
    <href>file.txt</href>
  </test>
  <test>
    <name>file1</name>
    <href>file1.txt</href>
  </test>
</root>
$VAR1 = {
          'test' => {
                    'file1' => {
                               'href' => 'file1.txt'
                             },
                    'file' => {
                              'href' => 'file.txt'
                            }
                  }
        };
<?xml version='1.0' standalone='yes'?>
<root>
  <test>
    <name>file</name>
    <href>file.xml</href>
  </test>
  <test>
    <name>file1</name>
    <href>file1.xml</href>
  </test>
</root>