Maybe you need Maybe?

subtype 'MyMatch',
      as 'Str',
      where { /$regEx/ },
      message { "$_ didn't match" };

has 'attr' => (
    is => 'ro',
    isa => 'Maybe[MyMatch]',
    required => 0
);
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See doc:

The Maybe[`a] type deserves a special mention. Used by itself, 
it doesn't really mean anything (and is equivalent to Item). 
When it is parameterized, it means that the value is either undef 
or the parameterized type. So Maybe[Int] means an integer or undef.
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Hope this helps!