After fiddling a bit with XML, I decided to take an easier approach, and decided to use YAML instead of XML. I now have a working test suite that tests against both the XHTML and the REST/YAML interfaces, using the same test data. I ended up just using LWP::UserAgent for that -- WWW::Mechanize wasn't necessary for testing of the REST/YAML side.
And I need to thank you for that -- you stated that XML wasn't necessarily part of REST, and you're right. Once again, Perlmonks provides great feedback, something that I rely on, since I'm the only Perl developer at my employer who is focussing on the web.