use strict;
use warnings;

use XML::LibXML;

my $xc =  XML::LibXML->new;
my $node = $xc->parse_string('<?xml version="1.0" encoding="UTF-8"?><xData><xID>28344</xID><mType>s</mType></xData>');

print $node->toString;

my $path = '//xID';
my $value = '123';
if (my $subnode = $node->findnodes($path)->[0]) {
    $subnode->removeChildNodes();
    $subnode->appendTextNode( $value );
}

print $node->toString;