OK. Agreed. 100%.
But I can't stop myself from thinking that there might or will be situations where $a and $b may be meaningful.
In a Windows specific application, when accessing diskette drives A: and B: it wouldn't be very far fetched to name two variables $a and $b. If I already know that it's a bad thing I wouldn't do it. But someone who doesn't know about the special meaning of $a and $b could find it quite natural to use those variable names in this situation.
Personally, I have today learnt about $a and $b for life. But if I could be unaware of their special meaning, many more people will. My co-worker, who definitively is a Perl beginner, had no way of understanding the behavior on his own.