As for whether or not, and if yes, how, you should use flock depends on what you are exactly doing. Normally you need to flock files if there's a change that if one process is modifying a file, another process is accessing the file (be it for reading or writing). The syntactical details about file locking can be found in perldoc -f flock and in man perlipc.

Abigail

Personally, for any bit of file handling code I write, I always get a shared lock if I'm reading it, and an exclusive lock if I'm writing/appending (unless this is just a quickie script that will never be used a second time). This does introduce a performance hit, but if that becomes an issue, it's a matter of commenting out one line.

I do this mostly as a means of covering myself. If someone else writes a program that also works with this file, and this file gets messed up because they didn't lock it, I can say: "I did my part. If you didn't do yours, that's not my fault."

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Reinvent a rounder wheel.

Note: All code is untested, unless otherwise stated