Once less has displayed the first screen full, it stops reading the pipe and waits for instructions from the keyboard. The pipe fills and won't accept anymore from the producer, so it blocks on the write until the pipe empties. Which it won't until you hit the space bar or similar.

buf.pl uses a shared queue as a buffer between two threads, The read thread just keeps reading and pushing to the queue. The write thread reads from that queue and writes to the pipe. Because the two are asynchronous, when the write thread blocks because the pipe to less is full, the reader just carries on filling up memory until the producer finishes.

The downside is that if the producer produces sufficient output, the perl process will eventually run out of memory and die.