$n = '4564123800603607';
# this is not my credit card number

($o = reverse($n)) =~ s/.(.)/($1*2)/ge;
# every other number in it, times 2 
# (reversed because then it will work 
# for numbers of different lengths, as
# some cards are 13, not 16).

($e = reverse($n)) =~ s/(.)./$1/ge;
# the other numbers, not times 2

$o =~ s/(.)/$x+=$1/ge;
$e =~ s/(.)/$y+=$1/ge;
# add the digits

printf("%sValid.",(($x+$y)%10 == 0?'':'Not '));
# print the result