gistfile1.txt

role R { };

my $x = 4 but R;
my $y = 2 but R;

multi sub foo ($a, $b) { say "foo without constraint" }

multi sub foo ($a where { .defined and .does(R) },
               $b where { .defined and .does(R) }) {
    say "foo with constraints"
}

multi sub infix:<cmp> ($a where { .defined and .does(R) },
                       $b where { .defined and .does(R) }) {
    say "cmp with constraints"
}

foo($x, $y);
$x cmp $y;