Is there any specific reason you want to use XML::XPath? XML::LibXML is very similar, except that it is a lot more powerful (supports XPath and DOM like XML::XPath, but also RelaxNG, catalogs, XInclude...), faster, and better supported. Its only possible drawback is that it is based on libxml2 instead of expat, which might be a tad more difficult to install if you are not on linux.
In any case you can see examples using both modules in the
Ways to Rome series of articles, and you can play with XPath at xpath_test.