I would assume that somewhere in MooseX::Types, it is declared how to pass an instance of a type or undef. Other languages use a Maybe(Str) construct for that, but I don't know what the Moose people believe should be used. Moose::Util::TypeConstraints claim that there is Maybe[`a], so maybe that's what you should use:

Maybe[Str] $foo
[download]

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.

Corion actually answered the question, but only by guessing, so I will restate his answer but with some more details and certainty.

You have two choices here really, the first is a type union, which is written as Str | Undef and this basically says that the value can be either a String or Undefined. The second choice is the Maybe[`a] parameterized type, which would be written like this Maybe[Str]. This gives you basically the same thing as the type union but communicates the relationship differently by more explicitly saying "this *might* be undefined" not "this *can also* be undefined". My personal choice is always to do Maybe[`a] instead of the union in cases like these.

Yes, thanks. I like the apparent semantics better of Maybe.