Here's the fastest and shortest way I could find to process an input file, and print the unique lines inside it:

Explanation: The ||= operator has a behaviour called "short-circuit". If the left-hand side of the operation evaluates to true, the operation will short-circuit and stop - essentially the right-hand side will be ignored and never evaluated. But if the left side is false, then whatever the right-hand side evaluates to will be assigned to the left.

In this case, we are processing each line from a file called "in". The first time we see $s{$_} it will evaluate to false, setting off the right hand side to be evaluted. This will print $_, return a 1, then set $s{$_} to 1.

The next time we see $s{$_} inside the loop, it is true, so the ||= never again runs the right-hand side operation. This results in the above code only printing a line the first time it's seen, and no more.