There doesn't appear to be any way to do what you want. The trouble being that Perl's subs are dynamic, so it is completely fair to have a sub that is created on the fly, and only exists when the program is running.

This is, after all, what happens when you import a module.

A quick browse of CPAN didn't show up any module that would do run-time checks for functions to exist, so it sounds like you have 'an opportunity, rather than a problem'.