Use XML::Twig:

use strict;
use warnings;
use XML::Twig;
use Data::Dumper;

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>
</root>
XML

my $twig = XML::Twig->new (
    twig_roots => {href => \&editHref,},
    twig_print_outside_roots => 1,
    );

$twig->parse ($xml);
$twig->flush;


sub editHref {
    my ($twig, $href) = @_;
    my $text = $href->text ();

    $text =~ s/\.txt$/.xml/;
    $href->set_text ($text);
    $href->print ();
}
[download]

Prints:

<?xml version='1.0'?>
<root>
  <test>
    <name>file</name>
    <href>file.xml</href>
  </test>
  <test>
    <name>file1</name>
    <href>file1.xml</href>
  </test>
</root>
<root><href>file.xml</href><href>file1.xml</href></root>
[download]

#!/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>
[download]

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]

Can any one clear my doubt, how to update the xml element values without changing the xml format if xml elements contains attributes like id, type also.?