$SEMAPHORE = $tempfile . '.lck';
  open(S, ">$SEMAPHORE") or die "$SEMAPHORE: $!";
   flock(S, LOCK_EX) or die "flock() failed for $SEMAPHORE: $!";
   
       open (TEMP, ">$tempfile") or die "Can't open $tempfile: $!";
        open (FH, "$file") or die "Can't open $file: $!";
          while (<FH>) {
               ## do_something;
          }
        close FH;
       close TEMP;
       rename($tempfile,$file);

   close S;