But my next goal was to round-trip the config file

My experience with XML::Rules was that, sadly, it is not good at round-tripping XML files (Update: see also Jenda's comments here). IMHO, if that's what you want, I'd just go with XML::LibXML, in conjunction with its XPath support it's not too difficult to pull values out of the config file while maintaining the original XML file's structure. An alternative might be XML::Twig, though my experience with that is more limited.