sub next_prime_iterator {
   my $num = shift;
   do { $num++ } until is_prime($num);
   return $num;
}

sub is_prime {
   my $guess = shift;
   return 1 if $guess == 2;
   return unless $guess > 2 and $guess % 2;

   my $max = 1 + int $guess ** .5;
   for ( my $divisor = 3; $divisor < $max; $divisor += 2 ) {
       return unless $guess % $divisor;
   }
   return 1;
}