use IPC::SysV qw(IPC_PRIVATE S_IRWXU IPC_CREAT);
use IPC::Semaphore;

my $sem = new IPC::Semaphore(IPC_PRIVATE, 1, S_IRWXU | IPC_CREAT);

$sem->setall( (0) x 10);

my $semid = $sem->getVal($sem);

if(my $pid = fork())
{
    $sem->op($semid, -1, IPC_NOWAIT);	#get the semaphore
    open(FILE, ">/myFile.txt") or die "error: $!\n";
    print FILE "Hello, my name is Perl.\n";
    close FILE;
    $sem->op($semid, 1, IPC_NOWAIT);	# signal semaphore
    $sem->op($semid, -1, IPC_NOWAIT);	#wait on semaphore
    open(FILE, ">/myFile.txt") or die "error: $!\n";
    print FILE "Hello, again, my child.\n";
    close FILE;
    $sem->op($semid, 1, IPC_NOWAIT);	# signal semaphore
}
elsif(defined $pid)
{
    sleep(3);
    $sem->op($semid, -1, IPC_NOWAIT);	#wait on semaphore
    open(FILE, "/myFile.txt") or die "error: $!\n";
    print while(<FILE>);
    close FILE;
    $sem->op($semid, 1, IPC_NOWAIT);	# signal semaphore
    $sem->op($semid, -1, IPC_NOWAIT);	#wait on semaphore
    open(FILE, "/myFile.txt") or die "error: $!\n";
    print while(<FILE>);
    close FILE;
    $sem->op($semid, 1, IPC_NOWAIT);
    exit(0); #exit child
}

$sem->remove;