Check out
XML::LibXML::SAX::Builder. It builds a DOM from a SAX stream. From there you can use XPath on the data set, and even serialise back to SAX when you're done if you want to. There's nothing in the SAX spec that says you shouldn't be able to use XPath on the data, or shouldn't build a DOM with it. SAX just happens to be the universal plumbing.
Also you might want to check out Barrie Slaymaker's XML::Filter::Dispatcher, which allows you attach to a SAX pipeline and dispatch to methods/functions based on XPath's specified in the constructor. Rather like XML::Twig, but better ;-)
I've been meaning to write a node about just how great SAX is for a while now, maybe it's time I just did it.