In English, saying "if this, do that, else do the other" makes sense and is easy for a person to parse. However "unless this, do that, else do the other" is much harder to parse because the "else" clause is a double negative (or even a triple negative if you have "unless not this"). Same applies to perl.

While you could, perhaps, argue that "if not this, do that, else do the other" is also a double negative, it's still easy to parse because it's familiar. We see it in every programming language ever.