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

my @buf;
my $file_count = 0;

while (<$in>) {
   push(@buf, $_);
   if (@buf == 30) {
      open(my $fh_out, '>', "file${file_count}.dat")
         or die("Unable to create file${file_count}.dat: $!\n");
      print $fh_out ($_) foreach @buf;
      @buf = ();
      ++$file_count;
   }
}

if (@buf) {
   open(my $fh_out, '>', "file${file_count}.dat")
      or die("Unable to create file${file_count}.dat: $!\n");
   print $fh_out ($_) foreach @buf;
}