I've been pondering a similar problem lately, so I'd like to piggyback on the OP's question. We currently use a messy mix of YAML, JSON, and other homegrown formats to store various pieces of configuration data in our app. We are planning on writing a couple of features soon which depend on representing the entire system configuration in one big text file. We're leaning towards XML, because of a similar requirement to the OP's: that the file be easy to read and manipulate in languages other than Perl.
I am slightly worried that we don't know what we're getting ourselves into, and that the one-big-file is going to wind up being just as much of a nightmare to deal with as our current hodgepodge of separate files. Can anyone suggest good resources for reading up on this? Is there a public library of standard XML formats, so that we won't have to re-invent the wheel? Is a well-designed DTD our magical answer for other languages? Has anyone traveled a similar path and learned any good lessons?