I like it the idea, a lot. XML::Composer sounds like a great name.
The only thing I'd love to see is a way of parsing XML-like data so that the crap you have to hand-roll a parser for is easier to work with. Though, I'm pretty sure this is beyond the scope of this module.
My criteria for good software:
Does it work?
Can someone else come in, make a change, and be reasonably certain no bugs were introduced?