Luhn test of credit card numbers

Here we make use of comb, which splits into individual characters, and the sequence operator ..., which can intuit an even or odd sequence from the first two values. %% is the divisible-by operator.

sub luhn-test ($number --> Bool) {
    my @digits = $number.comb.reverse;
    my $sum = @digits[0,2...*].sum
            + @digits[1,3...*].map({ |($_ * 2).comb }).sum;
    return $sum %% 10;
}

# And we can test it like this:

use Test;

my @cc-numbers =
    '49927398716'       => True,
    '49927398717'       => False,
    '1234567812345678'  => False,
    '1234567812345670'  => True;

plan @cc-numbers.elems;

for @cc-numbers».kv -> ($cc, $expected-result) {
    is luhn-test(+$cc), $expected-result,
        "$cc {$expected-result ?? 'passes' !! 'does not pass'} the Luhn test.";
}

Output:

1..4
ok 1 - 49927398716 passes the Luhn test.
ok 2 - 49927398717 does not pass the Luhn test.
ok 3 - 1234567812345678 does not pass the Luhn test.
ok 4 - 1234567812345670 passes the Luhn test.