use Fcntl qw(:DEFAULT :flock);
...
    sysopen(LOCKFILE,$lockfile,O_RDWR|O_CREAT) # create if necessary
        or die "Can not open/create $lockfile ($!)";
    flock(LOCKFILE, LOCK_EX) or die "Locking error on $lockfile ($!)";
    eval {
        my $number_of_clients=<LOCKFILE>||0;
        chomp($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: $@"; }
    flock(LOCKFILE, LOCK_UN) or die "Unlocking error on $lockfile ($!)";
    close(LOCKFILE) or warn "Error closing $lockfile ($!)";