#!/usr/bin/perl
use warnings;
use strict;

use XML::LibXML;

my $file_dati = '/path/to/file.xml';
my $parser    = 'XML::LibXML'->new();
my $doc       = $parser->parse_file($file_dati) || die('dead1');
my $xc        = 'XML::LibXML::XPathContext'->new($doc);

$xc->registerNs('mw', 'http://myweb.com');

my $nodetobefound = '//mw:comment[@timestamp="20130903225513"]';

print "To be found: $nodetobefound\n";

my $commento = $xc->findnodes($nodetobefound) || die('dead3');

print $commento->get_node(0)->toString();