It makes a difference only when $| was -1 beforehand. If it was zero, ++ makes it 1 and that's the case you're expecting. If it was 1, ++ makes it 2, which is still true, so it has the same side effect as $|=1 would. If it was -42, than it becomes -41 which is also still true. Only if it was -1 then ++ makes it false, which is indeed different than $|=1.

Therefore, they are not the same.

Update: Nevermind.