Perl one-liner, then :)

Looking back, my day 4 part 2 solution appears to be 4 lines of Perl ... I can see what it's doing, but I no longer understand its underlying algorithm or why it works.

This is why I spend a bit longer on my AoC solutions than many (I never get on the leaderboard really) but I try and write code I *can* come back to and understand later.