Well, you only need one number since the RSA numbers are factors of 2 primes.
Oh Yeah, and you only need every other number, since you know that it isn't prime if it's even.
Also, you don't need to count for 2, because you can check to see if the last number is even, and then the challenge wouldn't be very hard.

$p=<the number>;$r=3;while((p%r)!=0)r+=2; <==Unreadably

**READABLY**
$p=<the number>;
$r=3;
while((p%r)!=0)r+=2;
**READABLY**


Not such a bad solution character count wise.

Just Another Perl Backpacker