I've had a gander at XML::LibXML but cannot see how to code it to be real-world HTML tolerant (so I can test it and see how tolerant it is).

You can't. At least not in Perl. XML::LibXML uses libxml2, which does the XML, and HTML, parsing. That's what you would need to change.

For the record, when I wanted to add HTML parsing to XML::Twig, I looked at HTML::Parser, XML::LibXML and tidy, and settled on HTML::Parser as the most robust and easy to use solution to get well-formed XML out of random HTML.