in reply to Manipulating data from a file
It looks like you're doing pretty well on your own just from seeing the code change over a few minutes. It's not obvious that you need to extract array elements one by one. There are examples of how to pass parameters to subroutines here: perlsub. The math you have to do is just adding digits-- the trickier part is separating the digits, but you already know how to do that (though you may not know it). You can add them in a loop once they're split.
One suggestion I'd make that you probably won't catch on your own this time through, is that you should make your hash all one case and force the month name to that case for comparison. That way the input can be 'Mar', 'MAR', 'MaR', or any other case combination.
You might also rewrite how you read the file-- it looks from the assignment like you're supposed to do it a line at a time instead of grabbing all the lines at once into an array. A problem with perl is there's more than one way to do everything.
And just to help people who are going to add comments, you should put in a note like "edit: (stuff that I changed)" when you edit it. When you're changing the code, just add a little comment.