gist1.ext

pmichaud@kiwi:~/p6/rakudo$ cat y
class MyType {
    method postcircumfix:<( )>(MyType:U: $value) {
        say "coerced $value";
    }
}

say "MyType(3) = {MyType(3)}";

sub xyz($x as MyType) { say $x }

xyz(3);

pmichaud@kiwi:~/p6/rakudo$ ./perl6 y
coerced 3
MyType(3) = True
Unable to coerce value for '$x' from Int to MyType; no coercion method defined
  in sub xyz at y:9
  in block <anon> at y:11