Using MooseX::Declare you can...

use MooseX::Declare;

class Foo {
    method baz(Str|Undef $x) {
        say $x // '[undef]';
    }
};

Foo->new->baz($_) for undef, "hi", [], {};
__END__
[undef]
hi
Validation failed for 'Tuple[Tuple[Object,Str|Undef],Dict[]]' with val
+ue [ [ Foo=HASH(0x2645708), ARRAY(0x101e2...
  [+] Validation failed for 'Str' with value ARRAY(0x2105220) and Vali
+dation failed for 'Undef' with value...
[download]

MXD uses MooseX::Method::Signatures so I guess you'll be able to adapt the code above.