Logic::Kleene implements three-valued logic. For a long time, I couldnt imagine a practical use of this, but now I know one: user authentication. Currently, the authenticate method of Mojolicious::Plugin::Authentication returns true if the user authentication worked and false if not.

But there's a third possibility. He existed but did not authenticate. And presumably you would want different feedback to the user in that case. So 3-valued logic would be nice. But I think it's a bit fixed. Better just to return 0 for success and a status code for each form of failure?