The Perl documentation isn't aimed at small children. It's aimed at adult who are assumed to possess some learning ability. Including the ability to learn what a word or phrase means. (If the don't possess that knowledge, they'll struggle to learn any computer language). Perl documentation is peer-to-peer (or peer-to-soon-to-be-peer). A medical text book would use synchronous diaphragmatic flutter instead of "common childhood malady".
undefined behavior is not archaic, nor obscure. Enough people understand and use it that's idiom in its context.
If you want to cater for people who would have no idea what "undefined behavior" means, cannot grasp it from the context, and cannot search the web for it, write a patch against perlglossary. That's what it's for.