...
    my $dotlockfile = $lockfile . '.lock';
    my $lfh;
    
    # create the .lock with O_EXCL (Fail if the file already exists)
    while (!sysopen($lfh, $dotlockfile, O_RDWR|O_CREAT|O_EXCL)) {
        # some timeout checking code to get out this loop
        ...

        # wait for the existing .lock to go away
        sleep(1);
    }
    die "Can't create $dotlockfile" unless $lfh;
    close($lfh);

    # we got our own .lock, now continue with your lock code
    # no need to flock here

    my $number_of_clients = 0;

    sysopen(LOCKFILE,$lockfile,O_RDWR|O_CREAT) # create if necessary
        or die "Can not open/create $lockfile ($!)";
    eval {
        $number_of_clients=<LOCKFILE>||0;
        chomp($number_of_clients); 
        seek(LOCKFILE,0,0) or die "Rewind error on $lockfile ($!)";
        truncate(LOCKFILE,0) or die "Truncate error on $lockfile ($!)";
        if($operation='checkin') {
          ++$number_of_clients;
          ...
        } else {
          ...
          --$number_of_clients;
        }
        print LOCKFILE $number_of_clients,"\n"; 
    }
    if($@) { warn "Exception: $@"; }
    close(LOCKFILE) or warn "Error closing $lockfile ($!)";

    # delete if necessary (our .lock will keep others at bay)
    if ($number_of_clients <= 0) {
        unlink($lockfile) or die "Unlink error on $lockfile ($!)";
    }
    
    # now release our .lock
    unlink($dotlockfile) or die "Unlink error on $dotlockfile ($!)";