#! /usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use XML::LibXML;

my @xmls = (<< '__XML1__', << '__XML2__');
<?xml version="1.0" encoding="utf-8" ?>
     <propfind xmlns="DAV:">
       <propname/>
     </propfind>
__XML1__
<?xml version="1.0" encoding="utf-8" ?>
     <D:propfind xmlns:D="DAV:">
       <D:propname/>
     </D:propfind>
__XML2__

for my $xml (@xmls) {
    my $xpc = 'XML::LibXML::XPathContext'->new;
    $xpc->registerNs('x', 'DAV:');
    my $doc = 'XML::LibXML'->load_xml(string => $xml);
    say $xpc->find('count(//x:propname)', $doc);
}