sub is_prime
{
   my $num = shift;
   return if $num == 1;  # 1 is not prime
   
   die "usage: is_prime(NATURAL NUMBER)"
      unless $num =~ /^[1-9][0-9]*$/;
   
   for my $div (2 .. sqrt $num)
   {
      return if $num % $div == 0;
   }
   return 1;
}

my @numbers = 1 .. 20;

print join ";", grep { is_prime($_) } @numbers;