Just in case there is some confusion or misunderstanding of some of the Perl tricks used I better go through some of that code and elaborate on what's happening. Note that I've taken interesting lines in processing order rather that the order they are coded.

this is a little tricksy. It creates a new entry in %firstLines that contains the index to the new line as the value and is keyed by the unique part of the line contents. @lines in scalar context returns the number of elements in the array.

checks to see if we've already seen a specific line.

builds the multiple entries for duplicated lines. Note that $firstLines{$data} returns the index number that was stored earlier.

Thanks much for the added commentary! Now I think I actually understand what you did.