Atomic Gift Wrapping
Gift wrapping. You rarely want to do it by the Christmas Tree because you know, you just know that just when you're in the middle of wrapping that huge Dino-Rampage Total Combat Battlezone box, one of your child processes will innocently try to access the living room resources, see the half-ready present, go totally ape-boinker, and make every single day till Christmas a grueling hell. So instead, you usually wrap the presents in some dark corner of the house — usually the attic, the shed, or the secret room you built specifically for that purpose. Then, when the pile o' prezzies is ready, you carefully peek out in the hallway, make sure there is no living being in sight, rush down the stairs holding the loot with every prehensible limb available, and dump the whole thing under the tree. When the amazed kids ask how the glittery boxes got there? Well… Magic.
Atomic file writing? Exactly the same thing.
Most of the time, you can write files at your leisure, but sometimes you have other programs that can access it at any time, and you don't want them to end up bits and pieces. So you either begin to play with locking the file ("No one enter the room until I'm done!") or you write the new file in a different place, and when everything is ready, you do a quick switcheroo where you replace the old copy with its new incarnation. There is still a window where things can go wrong, but it's a minimal one.
Of course, the switcheroo is much easier (and funnier) to say than to do. That is, unless you use File::AtomicWrite, which takes care of all the nitty gritty details for you. To wit:
To accomodate different levels of paranoia, the module allows for several options, including the directory in which the temporary file is written, the template for the name of said temporary file, optional checksum of the data to be written, the minimal size that data should be (wishlist under 50K? IMPOSSIBLE!), and much, much more. But, and this is the good news, all the hard stuff has been taken off your plate. Now all you need to do is to write to the file (easy) or, in the real world equivalent, find the gift that needs to be appended under the tree (… okay, maybe not that easy).