XML (and, hence, XHTML) has two different levels of "correctness". First you can check that the document is well-formed. That means that it conforms to the syntax rules for an XML document (balanced tags, correctly formed attributes, that kind of thing). Then you can check that it is valid. This means checking it against a DTD and ensuring that only the correct tags and attributes appear (and in the right places).

If an XML document is succesfully parsed by XML::LibXML, then it is well-formed. It sounds like that's all you're interested in checking - therefore you don't need the validity check.