What you need to know about prototyping in general is don't. Except when it does something really useful for you.

The useful places are as follows. If you need to emulate a built-in, you have no choice but to prototype. If you want a constant to be optimized at compile time, you should use a prototype. (But constant does that for you.) The ($$) prototype is useful in sorting. And an & prototype can turn blocks into subroutines saving the need to type "sub ".

Personally the only prototype that I have found useful in the last 2 years is putting ($$) on a sort routine.