open(my $fh_in, '<', "filein.dat")
   or die("Unable to open filein.dat: $!\n");

my $fh_out;
my $file_count = 0;

while (<$fh_in>) {
   if ($. % 30 == 1) {
      open($fh_out, '>', "file${file_count}.dat")
         or die("Unable to create file${file_count}.dat: $!\n");
      ++$file_count;
   }

   print $fh_out ($_);
}