I use XML::Simple to generate/parse data streams for a Network Monitoring application. Since I have pretty much complete control over both ends of the pipeline, I can get away with very simple XML structures to pass data between my various plugins.

At the other extreme is a "display" generator for banking system, where I don't have control over what is coming into the pipe. The application reads a Data Stream and builds an XSLT template to parse and format the stream for an output 'device'. The device may be another application (with a different data structure), a browser (there are three accepted, 'company standard' browsers, each with its own set of quirks to be coded for), a backup device (with the data formated for storage efficiency rather than Human readability), a virtual device (I haven't a clue what is on the other end of the pipe, convert everything into cannonical form and ship), a plotter/printer (several kinds), etc.

This one started as an XML::Simple application and quickly got out of hand. The second system was based on XML::Tree, and it is starting to have some issues. We may have to fall back to building something unique from XML::Parser, but all the votes aren't in yet.