The monastery's authority for such questions is dave_the_m and IIRC we already briefly talked about introspection here.

Regarding a (robust) API:

IIRC this is depending on the version of Perl, early versions compiled code into the function block to expand @_, later versions had dedicated OP-Codes for that. ( I could dig up that info from emails, if Dave doesn't show up)

so this could either boil down to inspecting the OP-Tree with B::Concise or calling a method from B (on a B::CV node most likely)

In any case, B::Deparse is written in Perl, so you can always look into it and diff different versions to locate how the info is extracted.

Whatever the result it would be nice if such an API was added to Sub::Util when it's not experimental anymore.

HTH :)

PS: This message will self-implode ASA Dave shows up ;-)