On the threadA you are putting shift into $Qab. What is this doing? I am assuming it is to put the parameter into the sub's $Qab, but correct me if I am wrong.

Correct. A thread function is just a normal function that takes it arguments in the normal way.

Also what role does $Qab->enqueue( undef );

Once threadA has finished its loop; it queues undef before terminating. That undef causes the while loop in threadB to terminate; whence it then enqueues undef which cause the while loop in threadC to terminate.

The shift function in a subroutine takes the first item from the parameter list @_, which in threadA's case is the Thread::Queue object $Qab. It was put there by the threads->new(...) method call. threadB has two parameters, so he used an alternate syntax for that. He could have written:

sub threadB {
  my $Qab = shift;
  my $Qbc = shift;
...
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He puts the undef value in the queue as a way of indicating to whatever process uses the queue that it has reached the last value. When threadB's while loop gets this value, it will terminate. If this value was not there, the dequeue method would keep waiting for a value.