A factorial function in Perl6 using a [*] meta operator to produce the factorial product of a range, additionally the function parameter is given using a placeholder variable $^i

meta_factorial.p6

sub factorial {
    if $^i < 0 {
        fail "Not a Natural Number";
    } else {
        [*] 1..$i;
    }
}

use Test;
isa_ok factorial(-1), Failure, "Factorial for -1 fails";
ok factorial(0) == 1, "Factorial for 0";
ok factorial(1) == 1, "Factorial for 1";
ok factorial(5) == 120, "Factorial for a larger integer";