Here's how I would do it with XML::LibXML. The query searches for the TestElement with "Hello" and changes it to "Bye".

use strict;
use warnings;
use XML::LibXML;

my $parser = XML::LibXML->new({"encoding" => "utf-8"});
my $doc = $parser->load_xml(string => "<XML> <TestElement>Hello</TestE
+lement><TestElement>Some Other Element</TestElement></XML>");

my $qry = "/XML[TestElement='Hello']/TestElement/text()";
my ($node) = $doc->findnodes($qry);
if (defined $node) {
    $node->setData("Bye");
}
else {
    die "no match for $qry";
}

print $doc->toString(), "\n";
[download]

Output:

<?xml version="1.0"?>
<XML> <TestElement>Bye</TestElement><TestElement>Some Other Element</T
+estElement></XML>
[download]

Note that "Some Other Element" remains unchanged.