class T {

    BEGIN {
        my %calculator = (
            min => sub ($a, $b) { },
            max => sub ($a, $b) { },
            mean => sub ($a, $b) { },
        );
        my $set = %calculator<>:k.Set;
        has $.method where * (elem) $set = 'min'; # This version doesn't work
        # has $.t where { $_ (elem) $set } = 'min'; # This version works.
    }
}

T.new(:method('min')).t.say