Your key problem is that in the line:

<result xmlns="http://www.test.com/integration/integration">
[download]

The xmlns is declaring that everything contained in the result is in that namespace, so there are no, e.g., code_a named elements in the empty namespace, which is what your findnodes call is looking for. They are instead in the http://www.test.com/integration/integration namespace. As choroba notes, you'll have to register that namespace in order to be able to look for elements within it. So you'll need something like:

my $xpc = XML::LibXML::XPathContext->new($doc);
$xpc->registerNs('t', 'http://www.test.com/integration/integration');
foreach my $book_node ($xpc->findnodes('/t:books/t:book') ) {
    say "book name is " .$book_name->findvalue('/t:name');
}
[download]

See also XML::LibXML::Node