Further to this, OP may want to check out

    Writing to one file from multiple processes
    File locking
    What the flock!? Concurrency issues in file writing.

and there is a lot of useful discussion in

Just a something something...

In reply to Re^2: File content written by script and a called script by BioLion
in thread File content written by script and a called script by chilukuri_perl

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