The flock is only advisory - it is not guarranteed.

The best way to coordinate your programs on the same server is to use a proper interprocess locker, such as IPC::Semaphore (I assume that you are working on Unix systems), create a public lock, and have all the processes that want to read the file to obtain that lock before accessing the file.

If the file you are trying to access is on the network, then use a network locking mechanism, such as, say, IPC::Locker.

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