It won't make a miraculous difference, but you might try parsing each line only once at the top of the function, rather than re-parsing them each time, i.e.: 
my @lines = map { [ /.*?\t(.*?)\t(.*?)\t(.*?)\t(\d)/ ] } <IN>;
# ... rest of function

[download]
Regex matching can be expensive, so if you're doing the same match multiple times, it's usually better to do it once and save the results.

I also noticed you're using $average = $sum / $#things to take an average. Despite appearances, $#things isn't the number of @things. Instead, you'll want to use $average = $sum / @things, since an array evaluates to its length in a scalar context.

/s

In reply to Re: Re: A very odd happening (at least. . . to me) by educated_foo
in thread Processing large files many times over by dimmesdale

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