Create the lock file if it doesn't exist. Then lock it, exclusively (such as via flock). No matter how your process dies, the lock will be released when the process is no longer running. Trying to lock the lock file will fail only if another process is already running. You don't even have to worry about a PID getting re-used. Don't have code that deletes the lock file. For monitoring convenience, after locking of the file succeeds, write your PID to the file.

This also has the major advantage of preventing a classic race condition where you end up with two instances running.

Regarding the part of your comment regarding PIDs, I believe you can kill 0, $pid to determine if a signal can be sent to that PID. If so, then verify that the PID is for a process executing the same script/program (by parsing the result of a 'ps' command, or using a module such as Proc::ProcessTable).

# Untested
use Proc::ProcessTable;

my $tobj = new Proc::ProcessTable;
my $proctable = $tobj->table();
my $pid = 0;
for ( @$proctable ) {
    if ( $_->cmndline =~ m/$script_name/ ) {
        $pid = $_->pid;
        last;
    }
}
print $script_name, 
  ( kill 0, $pid ? q{appears} : q{does not appear} ), 
  q{ to be running}, qq{\n};
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Hope that helps.

What I'd really like to achieve would be a bulletproof lockfile that can withstand this kind of situation. I remembered that File::Temp has an nice UNLINK property that forces the file to be deleted when the process exits. I'm not sure if this happens when the process is killed, however. But that's the sort of functionality that I'm looking for.

There are two approaches for deleting temp files. The first is an END block that deletes the file (but that doesn't help in the case of power outage), and the second is to call unlink while the file is still open. on UNIX systems that gives hides the file from all processes, but it remains on disc until no more file handles to it are open. Of course that's no good for locking either.

Another approach to the lock files is to put them into a location that the operating system clears out at boot time (/var/lock/ on Debian systems), or to put them on RAM discs in the first place, whose contents automatically disappear at power down/reboot.

I realize that all of that isn't exactly what you're looking for, but maybe it still gives you some ideas.

Thanks for the great replies!

I think I'll use flock as tye suggested. Seems to do exactly what I'm looking for. I like the idea of using directory that gets cleared out on boot like moritz suggested as well. That was my initial thought for using File::Temp, to get access to the system specified temp directory.