An "upper base" ?? Somebody is going to have to explain that to me.

Well, geometrically it does perfect sense. It's part of the lexicon, I'd say. But if you like, the buzzword is oxymoron, which of course is not an oxygenated moron.

Is it like an "internal exoskeleton"

Oh, well hypothetically an organism could have an "internal exoskeleton", were it covered with further soft tissues... but... wait... we should call it "mesoskeleton" then... what a headache... what was the Perl question anyway?!? :-)