Biut Perl, K, Haskell and its friends, even colorForth, they are trying to solve real problems. They might be using odd ideas to do it, or they might look inaccessible, but that is not the point, they are honestly trying to be useful.
Esoteric languages like Piet, they aren't solving a problem. They are just being obscure for the sake of being obscure. This isn't interesting, and it doesn't really teach you anything of value.