Lately, I've been dealing with large, complex data structures using XML. Mostly I'm trying to teach myself the mysteries of XML and XSLT and determine which of the various XML-related Perl modules I like the best ;) And because most of my programming provides a back-end for dynamic web pages, the XML/XSLT combination has proven ideal.
One of my current projects involves a monstrous database of authors and books including with just about any information relating to them I can imagine. I'm pulling the data from several different tables and smashing it together into a single XML structure that can be relatively easily made into a web page using XSLT. For me, using XPath to pull out pieces of the data structure to manipulate feels more intuitive than a complex set of hashes and/or references.