> I don't have any code to really post, because I can't think of how to start this one...

<hint type="broad"> Try looping over your input file, collecting the data in a hash, and analyzing it.</hint>

If you then post the code here, and explain exactly where you're getting stuck (this assumes that all your questions haven't yet been answered - although effort is usually the thing that resolves questions), you'll find a number of capable people willing to help.

Please note that "help" here on PerlMonks is not defined as "we'll do it for you". It is, usually, defined as "you do it, and we'll assist as necessary."

-- 
Human history becomes more and more a race between education and catastrophe. -- HG Wells