zwon gave my first look reply. However I'm curious about your else if construct. Is that a typo (which implies you didn't copy and paste actual code), or haven't you bothered to tell us about the actual problem you are having?

Maybe if zwon's answer isn't the solution you are looking for you should reply with some code that demonstrates the error you are getting and (for some very small sample data set) give an expected answer. Oh, and always use strictures (use strict; use warnings;).