Tongue in cheek: clearly it must be XML-derived, have well-defined DTDs, written up as a RFC, have provable and hashed content to avoid contaminations, and signed by a trusted certificate authority. Being able to fetch an initial local config from an https repository is a plus.

I think things are often over-designed, even in the Perl world. Think of effective design, not capable design.

It's not bad to use something simple and "old-fashioned" that works, as long as you isolate your design decisions so they can be improved later. Keep a mind on the possible security implications (such as how someone can inject new code into a string that gets eval treatment).