I think choroba was suggesting this (untested):

$sem = IPC::Semaphore->new( 4321, 1, S_IRUSR | S_IWUSR | IPC_CREAT | I
+PC_EXCL );

if ( $sem ) {
    # New semaphore
    print "Semaphore created\n";
    $sem->setval(0,1);
    print "Semaphore initialised\n";
} else {
    # Semaphore already exists so just open it
    print "Semaphore already exists - just open it\n";
    $sem = IPC::Semaphore->new( 4321, 1, S_IRUSR | S_IWUSR );
+ );
}

print "Locking other threads\n";

$sem->op(0, -1, SEM_UNDO);
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But there is still a race condition, where the second process can attach to and use the semaphore after the first process creates it and before the first process initializes it.