The reasons for this is already thouroughly explained, so I just want to add a word of "wisdom"... or something.
In any programming generally, you should really use describing variables, because, much like "use strict", it will save you a lot of grief down the road. In perl, specifically, it also makes sure you don't clash with any special variables. Most of the time, people just don't name their variables $" or $\, even when not being verbose, so it is rarely a problem - apart from a few cases like this one.
I do understand that this was only test code, but it doesn't hurt to use names like $first_element and
$second_element or something, anyways. And, use lowercase variable names, so it will never clash with for example the names from "use English".
You have moved into a dark place.
It is pitch black. You are likely to be eaten by a grue.