The reasons for this is already thouroughly explained, so I just want to add a word of "wisdom"... or something.
in reply to use strict won't require explicit name for all variables?
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.