rather not use modules, sorry. i want to be able to write it myself.

Sure, it's a nice learning experience to write something yourself. Two comments:

• format isn't a module. It comes with the perl core.
• It's usually a waste of time and resources to solve a problem that has already been solved. If a module is readily available on CPAN, it's usually a good idea to use it, because chances are that it has been community-tested (no guarantees, though) and it's difficult to come up with a better solution if you start from scratch.