use strict;
use warnings;

use List::Util qw/sum/;

my $test_number = 123456789012;

$test_number =~ m/^\d{12}$/ or die "Invalid code.\n";

my( @even ) = reverse( $test_number ) =~ m/\d(\d)/g;
my( @odd  ) = reverse( $test_number ) =~ m/(\d)\d?/g;


my $checksum = sum( @odd ) * 3 + sum( @even );

print $checksum, "\n";