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