A Perl6 factorial function using a switch (given/when) statement and recursion.

given_factorial.p6

sub factorial (Int $i) {
    given $i {
        when $i < 0 { fail "Not a Natural Number" }
        when 0      { 1 }
        when 1      { 1 }
        default     { $i * factorial $i-1 }
    }
}

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";