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]