what is the mechanism for adding new routines which are to act like built-in functions if parameter checking is desired?

Prototyping, as I said.

As it is, prototyping has limited appeal if it is restricted to same file usage.

There is no such restriction.

While I said that require fires too late for perl to do anything with your prototypes, I also gave you a suggestion on how to require your other files in time for perl to check the prototypes on your functions. Give it a whirl; I think you'll find that it works as you expected your first attempt to work.