ikegami really proposed the easiest solution; file line number (represented in $.), modulus 16. If there's no "remainder", you're on a line that is a multiple of 16. You could, as an alternative for math-challenged individuals, increment a counter, and when it gets to the trigger value reset it and print the line. But that's more work just for the sake of being lazy. ;)

Oh, that reminds me of why I replied to this post (after an obviously perfect answer has already been given)... You are asking how to read every 16th line, but you really should be asking how to ignore all but every 16th line. Or in other words, read every line but only take action on multiples of 16. There is no easy way to only read every 16th line, if lines are of unknown length. That's a little like asking how to find "steam" in the dictionary without thumbing through it to find it.

There is the other lazy and inefficient alternative of using Tie::File to gain an index to each line, but then you still have to do math, so it really doesn't buy you anything other than the ability to easily access any line at will.