4. Semaphore Files

         $FILE = '/data/hitcounter';
         $SEMAPHORE = $FILE . '.lck';

         print "Content-type: text/html\n\n";

         open S, "> $SEMAPHORE" or die ...;

         if (something()) {
           flock S, LOCK_SH;
           open F, $FILE or die ...;
           print while <F>;
         } else {
           flock S, LOCK_EX;
           open F, "> $FILE" or die ...;
           my $data = qx{some command};
           print F $data;
         }

         close F;  close S;

This solves all sorts of problems
I had a long conversation last week with a guy trying to lock a DB_File database portably
- Believe me, you don't want to hear the details
- The upshot: This trick works, and nothing else does