It seems the variable as seen my the subset declaration is undefined. Here's a simplified example:
> my $must_be='b' b > subset OneStr of Str where $_ eq $must_be; (OneStr) > my OneStr $x = 'b' Use of uninitialized value $must_be of type Any in string context I don't know if that's a bug in Rakudo, or if it makes sense in some bigger picture.
