Problem or not problem. I consider the usage of certain variable names as 'special' variables a problem.
Will $nbr be used as a special variable one day? Or what about $i if we stick to one character names? (Wonder how many scripts would be broken by that last one.)
OK, so $a and $b were both there before me, but how am I to know? How am I to know that I need to read up on sort to know that they exist?
I admit that there are $_ and many other 'special' variables and I use them on a daily basis. But they look different and I instinctively don't use those names as my working variables. With $a and $b I have to explicitly be aware of their special behavior/existence or else devine their existence which is very far from intuitive.
But that is, of course, just an opinion. ;-)