Hello again, I have finally used JavaFan's code, changing it slightly to get an output similar to Nkuvu's, and managed to package it with PAR to get a standalone program. I can say it works flawlessly even with very large files with many thousands of lines. Thanks again. However, I still do not understand it thoroughly. I think I understand everything except

    push @{$reverse{$value}}, $key;
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What exactly happens here for each pair of keys and values of my first hash? I can see that, after that, in those cases where the value is the same for two or more keys, those two or more keys are contained in one "key array" (?) (one line if I print it out), and subsequently split with local $". But I do not really understand the above line of code. If JavaFan or someone else could explain it with words I would very much appreciate it.