use XML::XPath;

use strict;
use warnings;

my $xml = do {local $/; <DATA>};

my $xp = XML::XPath->new( xml => $xml );

$xp->setNodeText('/XML/TestElement', 'Bye');

print $xp->findnodes_as_string('/');

__DATA__
<XML> <TestElement>Hello</TestElement> </XML>