lass Dog {
    has Str $.name;

    method bark(Dog:D $dog: Int:D $x) {
        say $dog.^name;
        say $dog.name;
        $dog = 'hello';
        say "bark" for 1..$x;       
    }
}

Dog.new.bark(3);

--output:--
Dog
(Str)
Cannot assign to an immutable value
  in method bark at b.raku line 19
  in block <unit> at b.raku line 24