There is no namespace declared, not sure how to handle that...

So the elements belong to the null namespace, which is the one XPath engines (should*) look into for a nodetest with no prefix.

If all the elements are in the null namespace, you may use XPC as you used it

my $xpc = XML::LibXML::XPathContext->new($root);
$xpc->findnodes(...)
$xpc->findnodes(..., $topic)
[download]

or you may avoid using XPC completely.

$root->findnodes(...)
$root->findnodes(..., $topic)
[download]

* — In practice, many XPath engine get this wrong. LibXML gets it right.