use XML::LibXML qw();
use XML::LibXML::XPathContext qw();

my $document = XML::LibXML->load_xml(location => 'Datei.xml');
my $xpc = XML::LibXML::XPathContext->new($document);
$xpc->registerNs(ns => $document->documentElement->getAttribute('xmlns'));

foreach my $ZN ($xpc->findnodes('//ns:Liste/ns:Listenelement'))
{
	print $xpc->findvalue('//ns:Nummer', $ZN);
}