The point of optional, gradual typing is to offer optional type safety. If I say "String", it must be defined and have an appropriate string value. Thus, undef would cause an exception. I would suggest that any non-overloaded reference would also throw an exception (No more printing out ARRAY(0x7fecea803280)). However, if stringification is directly overloaded, we'd have to respect the intent because printing DateTime->now is intended to work. The integer 42 would need to stringify to 42 because even Java allows this as the one case of operator overloading because it's so damned useful.
That being said, I could be smoking crack. And I'd want these to be exceptions, not warnings, because I find so many ways in which warnings can be overlooked.