Dominus has a few slides about this in his Perl Hardware Store talks. The relevant slides start here.

Arjen

use Fcntl; #contains codes to use for flock
...
open(STORE,">$storable_file") || die "Could not open $storable_file: $
+!\n";
flock STORE,LOCK_EX;
store_fd \%hash,\*STORE;
... # code code code
flock STORE,LOCK_UN;
close(STORE); # closing file also unlocks it, but I prefer explicit un
+locking
[download]

As I pointed out here, you shouldn't use LOCK_UN, just close the filehandle to the lockfile (in this case it's the data file too).

Also, you should check the return value of close calls.

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If the information in this post is inaccurate, or just plain wrong, don't just downvote - please post explaining what's wrong.
That way everyone learns.

flock can fail (die) as can close (warn)

I'm not quite sure what while I do all of the reading/writing I need to do means

What I ment was that I need to do more than just read or just write the file, I need to be able to read the file, make changes, then write back. If I lock the conventional way, then I'll open a file handle for read, lock it, release the handle, open a handle for write, lock it, then release the handle. There is a period of time between my two locks where the file could be changed. That means the version of the file in memory would be different then the one on disk (the version in memory would be older).

If I make changes to the version in memory then write out to disk, I'll clobber changes made to the file after I read it into memory. So I need a lock that will let me have exclusive access to the file until I'm done with it.