After much muttering back and forth between blech and I, I have written and released AudioFile::Identify::MusicBrainz. The old MusicBrainz client relied on the C library they distribute, which was silly for something that sent and recieved pure RDF, so a pure-perl implementation was just begging to be written.
So, upsides, it works, there's exactly one sort of query you can send it, but that's fine, it's the important one - you can say 'I know this about a track, tell me more'. The next step is to give the user a nice choice, and then let them write the updated information back into the ID3 tag.
Downsides, I'm not using 'real' RDF parsers, I'm using XML::DOM. This worries me, frankly, I'd much rather do the right thing, but I get a headache trying to make the perl RDF stuff work. There's an RDF::Simple out there now, though, so maybe I'll try that...
I'm going to get a reputation here for stupidly long module names, you know.