And, copy on write has almost zero benefit in perl scripts
Well, there's nothing to argue about here. I was not talking about benefits for perl scripts. I've just mentioned existing feature. So, you are right when you say that it does not bring much of benefits for perl because perl scripts are not in code segments but in data segments.
Actually, Perl's threads are very different from your description
It all depends on what is "difference". I only mentioned basic idea of thread vs process. That idea stays the same: thread shares memory of process with other threads. Of course there are different strategies on how that memory is shared and particular sharing may only worsen memory consumption at the end.
The bottom line is that a working knowledge of the basic executable structure doesn't tell you much about Perl processes and almost nothing about Perl threading.
That is true. I just didn't see that the OP wanted to have knowledge about Perl threading. I thought, that the question was more about memory management for Perl. So I pointed out, that perl scripts are loaded into data segment and they don't get shared like segments of perl itself. If I misunderstood the question, then sorry.