I'm very much an XML monk, whenever I read grammar/structure I tend to go for an XML approach. Of course it can be overkill..., it does give you rich ways of defining your own languages/grammars and validate them with standard tools. It's not easy to match that kind of functionality.

Some pointers that might be of help to do it non-XML:
Data::Schema (and related modules) and Data::Validate::XSD (don't be mislead by the XML-ish name)

HTH
dHarry