I don't think my perl skills are that bad these days, but I have a terrible mental block thing going on with hashes. I want to be able to do something now that I know can only be sensibly done with a hash.
This isn't the actual problem - but it explains what I want to do. I just need a kick in the right direction so that I can implement this in my own script now and others in the future.
Imagine I have a text file that looks like this. 
HIT: code2 
HIT: code3
HIT: code1
HIT: code90
HIT: code2
HIT: code34
HIT: code90

[download]
I would like to be able to create a hash that, for when, for example, code90 is first hit, its 'counted' (i.e. a variable is increased by one) and goes into the hash and is stored. As I am only interested in whether code90 exists, rather then how many times it exists, if I then encounter it again, I need to check the hash to see whether I have already counted it. If I have, I want to ignore that entry and move onto the next in the text file.
How do I go about doing this please?
Any help much appreciated.

In reply to help on how to create a hash look up table requested. by Angharad

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