In general, I'd try using something like:

constraints => {
  myfield => { 
    constraint => sub { return (checkField() ? 1 : 0) },
    params     => [qw(myfield, otherparam1, otherparam2)],
  },
  ...
}
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...where you can either use a subroutine checkField() or replace it with a short test right there in the anonymous subroutine.

There is an example in the documentation for the module, but it uses a built-in validation routine rather than an anonymous subroutine...

buckaduck