If someone did repplace can(), he must know what he's doing.
Yes, but your code won't let him do it. If I'd written a plugin with a custom can(), to make it work with your program, I'd have to override UNIVERSAL::can()! Putting specific code to make a subclass work in a superclass (let alone the ultimate superclass) is bad design.
I understand that your code tries to ensure that everything in @plugins actually represents a valid invocant. That's well-worth doing, but I think you're forcing behavior of the plugins beyond the minimum you need to do what you're doing.
You've created a plugin system because you don't know what people will need to do to solve problems in the future. Don't enforce too many restrictions on how they'll solve those problems!