Are those really your variable names?

If so then I'd strongly recommend that you switch to more verbose and descriptive names. That might have solved this problem, because it is a lot easier to miss that you put the wrong letter somewhere than it is to miss the fact that you have the wrong name. (And a typo will result in a variable that you didn't use - which strict.pm can catch.)

Furthermore note that $a and $b are reserved for sort, so you should definitely use something else instead of those two.