You may want to take a look at Watchdog::Process. I've never used it before, but 'watchdog' is normally what you call a process that monitors another one, and a quick search turned that up.

Update: Well, if that one's out, I can offer the following logic, that I wrote ~5 years ago to sense if the process I was called was already running. it was written for Solaris, but you can change the arguments to ps to fit for BSD.]

sub check_running {
    my $return = `ps -ef | grep process_incoming | grep -v grep | wc -
+l | tr -d '[:space:]'`;
    # blah...running from cron, there's two processes -- sh, and perl:
    if ( $return != 2 and $return != 1 ) {
        # wrap this in an eval to keep it quiet
        # (or the 'die' will generate output)
        eval {
            &log_error("Aborting -- 'process_incoming' showing in ps  
+[$return]");
        };
        exit;
    }
    return;
}
[download]

Okay, I admit...it was from a shell script originally, and I just recycled the same command, rather than spending the time to figure out how to track processes from within Perl.