If you want mutating on an object, than you should define method to work that way already.
No, that is exactly the opposite of what I want. That way you get again the mess that Perl 5 also has: some are mutating, some are not. I want everything to be non-mutating by default with some kind of modifier to make it mutating. This thread made me think of .= and I think it is a very good candidate, because it works much like the other op= operators: foo op= bar does the same as foo = foo op bar, but with possible optimizations.