I'm the person that posed the question to beppu ;)
Thanks for the great answers, I'm glad I finally made it on to perlmonks!
So... Proc::Podfile didn't work out for me too well. I couldn't seem to get it working right. I might have been using it wrong, or perhaps I just didn't understand how to use it from reading the docs.
Then I found File::Pid, and I'm very happy. Unfortunately, it is not smart enough to release the pidfile if the script crashes, but that's not a big deal for my current needs.
Here was the implementation I used. I decided to allow my script to get a --force flag, in case it was really important that the script run at a certain time (even if a crash happened and there's a pid file hanging around). Under normal circumstances, the script will exit if another instance is discovered.
use File::Pid;
# Check to make sure we are not already running
my $pidfile = File::Pid->new({file => "/path/to/my.pid"});
# This next line gets the current pid in $pidfile
# If nothing was running, we should get back our own pid
my $pid = $pidfile->write;
# now, die if the pid file cannot be opened,
# or the pid running is not THIS INSTANCE
# note: this can be overridden if $FORCE is true.
die ("running process=".$pid.", my pid=$$\n") if (!$pid or ($pid != $$
+ and !$FORCE));
# ... do a bunch of stuff for a long time
$pidfile->remove or warn "Couldn't unlink pid file\n";
somebox |