Thanks for the comments! A RDBMS it will be. I will definitely give XML::Twig a try. It has been on my "to do" list for some time.

Wrt the validation it ranges from simple (i.e. number or date in a certain range) to complex (Right- Ascension/Declination of a satellite orbiting some celestial body at a specific time is “correct”). It cannot be captured by XML Schema or any other schema language. We have lots of IDL code and third party libraries to validate the data.