my $block_size = 1000;
my $max_line_size = 10000;

my $buf    = '';
my $offset = 0;

my $read;
my $line;
my $pos;

READ_LINE:
for (;;) {

   EXTRACT_LINE:
   for (;;) {
      $pos = index($buf, $/);
      if ($pos >= 0) {
         $line = substr($buf, 0, $pos+1, '');
         $offset = 0;
         last EXTRACT_LINE;
      }

      FILL_BUF:
      for (;;) {
         my $to_read;
         if ($offset + $block_size > $max_line_size) {
            $to_read = $max_line_size - $offset;
         } else {
            $to_read = $block_size;
         }

         if (not $to_read) {
            SKIP_LONG_LINE:
            for (;;) {
               $read = read($fh, $buf='', $block_size, $offset=0);

               die("Unable to read: $!") if not defined $read;

               if (not $read) {
                  $line = undef;
                  $offset = 0;
                  last READ_LINE;
               }

               $pos = index($buf, $/);
               if ($pos >= 0) {
                  substr($buf, 0, $pos+1, '');
                  $offset = $read - ($pos+1);
                  last SKIP_LONG_LINE;
               }
            }

            next EXTRACT_LINE;
         }

         $read = read($fh, $buf, $to_read, $offset);

         die("Unable to read: $!") if not defined $read;

         if (not $read) {
            if (not $offset) {
               $line = undef;
               $offset = 0;
               last READ_LINE;
            } else {
               $line = $buf;
               $buf = '';
               $offset = 0;
               last EXTRACT_LINE;
            }
         }

         $pos = index($buf, $/, $offset);
         if ($pos >= 0) {
            $line = substr($buf, 0, $pos+1, '');
            $offset = 0;
            last EXTRACT_LINE;
         }

         $offset += $read;
      }
   }

   ...do something with $line...
}