I might be wrong, but I think the technical terms for overwriting functions are

• overriding for built-ins
• monkey patching for imported functions and methods

I'd use overloading explicitly for perlops

But this might be restricted to Perl lingo and different in other languages (the line between operators, built-ins and functions is blurred and depends on context)

And I have to admit that I'm anal about terminology, a "maladie professionnelle" of studying math...