in reply to Learning to use the hash effectively
One way to fix this is to open the file in read/append mode, flock it, seek to the beginning of the file, read from it, alter the file contents in memory, seek back to the beginning, truncate it, then rewrite the contents from memory. The flock will prevent the race condition (at least w/ another version of your program that uses flock).
Another way to fix the problem is to use a semaphore file, like in tilly's Simple Locking. Here, you flock a semaphore file when you want to enter the "critical section" of your program, and then other processes of your program cannot enter that critical section until you have released the lock.
I would recommend the second approach.