my $parser = XML::LibXML->new();
my $doc    = $parser->parse_file($file_name);
my $root   = $doc->documentElement();

my $xpc = XML::LibXML::XPathContext->new();
$xpc->registerNs( xxx => 'http://xxx' );

for my $node ( $xpc->findnodes('/xxx:xxx/xxx:yyy', $root) ) {
   ...
}