See doc for findnodes, you need to bind a prefix to the namespace (xmlns), so

my $xpc = XML::LibXML::XPathContext->new;
$xpc->registerNs( 'x', 'http://www.inktomi.com/' );
$xpc->findnodes( '/x:ysearchresponse/x:resultset_web/x:result/x:url', 
+$doc )

$xpc->findnodes( '*/*/x:result/x:url', $doc )
[download]