How to write an xpath query including attributes for an XML with namespace

[topola1882]

Hello everybody, i think i have a simple question but i can't find an aswer to it anywhere so i hope you can help.
I have an xml file with namespace and i want to select a node with an attribute with a specific value. Here is the xml file:

<?xml version="1.0" encoding="UTF-8"?>
<guestbook xmlns="http://myweb.com" xmlns:xs="http://www.w3.org/2001/X
+MLSchema-instance" nome="740" xs:schemaLocation="http://myweb.com gue
+stbook.xsd">

<comment confermato="true" timestamp="20130627112554">
  <guest>...</guest>
</comment>
<comment confermato="false" timestamp="20130903225513">
   <guest>...</guest>
</comment>
<comment confermato="false" timestamp="20130904191337">
   <guest>...</guest>
</comment>
</guestbook>

I need, with a cgi script, to select the node "comment" with the timestamp "20130903225513". The problem is that i don't know hot to put the namespace correctly. Here is the code for the script

#!/usr/bin/perl

use strict;

use XML::LibXML;
use HTML::Entities;
use CGI;
use CGI::Carp qw(fatalsToBrowser warningsToBrowser);
use Fcntl qw(:flock);

my $cgi = new CGI;
print "Content-type: text/html\n\n";

my $file_dati = "../data/xml/guestbook_xml.xml";
my $parser = XML::LibXML->new();
my $doc = $parser->parse_file($file_dati) || die("dead1");
my $root = $doc->getDocumentElement || die("dead2");

my $nodetobefound= "xs:commento[xs:\@timestamp='20130903225513']";

print "$nodetobefound\n";

my $commento = $root->findnodes("$nodetobefound") || die("dead3");

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

If i don't insert any namespace in the query i finish in the "dead3" die, if i put any kind of namespace i receive an "Xpath error: invalid expression".
Any hint on how to write the correct xpath query?
thanx in advance, Giulia