Well, try to explain $_ in a short sentence without being wrong. "It's the perl equivalent of the pronoun 'it'." OK, now try explaining it in proper engineering terms that describes exactly what it is, how to use it, and in some sense, how to implement it, in about that many words.

At the very least, I suspect honesty will compel you to admit that either A: that took you longer than you expected or B: you left out a lot of corner cases, especially when modifying $_ does or does not change the thing it does or does not reference, and how it interacts with local, and, and, and....