You might want to start with Simple Module Tutorial, authored by the great tachyon. You may also find merlyn's book, published by O'Reilly, called "Perl Objects, References and Modules" helpful, as I did.

As for efficiency, that has a lot to do with what you're trying to accomplish, and what algorithms you use to accomplish the task at hand. We can't be very helpful in that respect without knowing what kind of a module you have in mind.