Thanks for admitting this is homework. It's a refreshing breath of honesty for such questions.

I don't know where to start or how to start

Start with the links that marto just gave you, combined with the textbook or class notes from the class you are taking.

1. Figure out the math for how to solve that problem manually. It is a simple math problem, and if you are advanced enough in your education to be taking a class on Perl programming, you should be able to solve it. If you need help with the math, perlmonks isn't really the place, because that's a math question, not a Perl programming question.
2. Translate that math into Perl syntax. Your textbook or class notes should have examples of how to do the simple math functions required.