my $file_name_in  = 'infile.txt';
my $file_name_out = 'file%d.txt';
my $max_lines     = 800;

open(my $fh_in, '<', $file_name_in)
   or die("Unable to open input file \"$file_name_in\": $!\n");

my $fh_out;

while (<$fh_in>) {
   if (($. - 1) % $max_lines == 0) {
      my $file_num  = int(($. - 1) / $max_lines) + 1;
      my $file_name = sprintf($file_name_out, $file_num);
      undef $fh_out;
      open($fh_out, '>', $file_name)
         or die("Unable to create file \"$file_name\": $!\n");
   }

   print { $fh_out } $_;
}