...

open(PIDFILE,">>$pidFile") or die
     "Cannot open pidfile, $pidFile: $!\n";

if (! &lock(*PIDFILE) )
{
    print "\n\tCannot obtain exclusive lock on $pidFile\n",
          "\tThere must be another instance of this\n",
          "\tprogram running.  Exiting.\n\n";
    exit;
}

# GOT LOCK
my @arr = (<PIDFILE>);
open(PIDFILE,">$pidFile");

print PIDFILE "$$\n";

...