Reading UNIVERSAL it says that ->does(Role) should always be the same as ISA. I think it will be more as well, but it will return true for anything isa would return true for. It also says that you have to override does for your module. So I think it will be a way for one class to say it does a certain set of methods. I.e. if you have a Logger base then anything that does(Logger) would provide a ->log method. Then things wouldn't have to inherit from Logger to be considered a Logger.