The main problem is that you add not only the link to the schema, but also a namespace to your document. To handle documents containing namespaced elements, you have to use XML::LibXML::XPathContext:

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

use XML::LibXML;

my ($userok, $passok) = (1, 0);

my $digestuser = 'dado';
my $digestpass = 'cane';

my $file   = 'user.xml';
my $parser = 'XML::LibXML'->new;
my $doc    = $parser->parse_file($file);
my $root   = $doc->getDocumentElement;

my $xpc = 'XML::LibXML::XPathContext'->new;
$xpc->registerNs('p', 'http://www.progetto.com');

for my $u ($xpc->findnodes('p:userlist/p:user', $doc)) {
    print "inside-cycle";
    my ($property) = $xpc->findnodes('p:username', $u);
    if ($property->textContent eq $digestuser) {
        print $userok;
        ($property) = $xpc->findnodes('p:password', $u);
        if ($property->textContent eq $digestpass) {
            $passok = 1;
            print "i did it";
        }
        last
    }
}
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The wrapper XML::XSH2 simplifies the code to

open user.xml ;
my $digestuser = 'dado' ;
my $password   = 'cane' ;
my $passok ;
if p:userlist/p:user[p:username=$digestuser and p:password=$digestpass
+] $passok = 1 ;
[download]