Another thing I like to do when creating a separate class/module for different formats is to add pod to the module to document the details of the format and what the fields are (and their start/end position and length). Then the documentation for the format is just a 'perldoc Something::SomeXFormat' away (and I don't have to hunt for the hardcopy I worked from to create the module) :-)

Update: What would be nice is if the code defining the format was actually read from the pod (or vice-versa), so the docs would be self-updating...but I'm not sure about the best way to do that at the moment.