To use a namespace in an XPath expression, you have to register it first. The following works for me:
#!/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();
As Corion suggested, I changed the name of the element and added // for recursive search.