use XML::LibXML;
use XML::LibXML::XPathContext;

{
    my $xml = <<'XML';
<?xml version="1.0" standalone="yes"?>
<sdnList xmlns="http://tempuri.org/sdnList.xsd">
  <sdnEntry>
    <lastName>Hello world!</lastName>
  </sdnEntry>
</sdnList>
XML
    my $parser = XML::LibXML->new;
    my $doc    = $parser->parse_string($xml);
    my $xc     = XML::LibXML::XPathContext->new($doc);
    $xc->registerNs('sdnList', 'http://tempuri.org/sdnList.xsd');
    my $result = $xc->findvalue('//sdnList:lastName');
    is( $result, "Hello world!", "Namespace" );
}