Since XML is just text -- why not search for text compression? But to answer your question -- yes this seems very possible. But is it practical? What I don't understand, however, is why you give the users the ability to alter the contents in the first place. My solution would be to prevent them from having access to the files.

But search CPAN and see if one of the many archiving and compression modules suites your needs. Feel free to ask specific questions about which ever module you pick if you run into troubles (installing, using, etc.).

UPDATE: I like CountZero's suggestion. :)

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)