4. Semaphore Files

         use Fcntl ':flock';

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

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