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open(PID, "> $CONF{upd_lockfile}") ;
flock(PID, 2);
...
    print(PID "$a\n") if ($a != $$);
}
flock(PID, 8);

use Fcntl qw( LOCK_EX );

...
   kill(CONT => $parend_pid) or die $! if !@pids;
   close($PID) or die $!;
}