I don't recall this caller-based technique to enforce privacy as being used much,
My personal opinion is that most people don't feel they "need" it. This is back to the question of "culture" vs "control". In my inside-out objects talk, I use the analogy of double-yellow lines on the road versus concrete lane barriers. (Apologies to non-US drivers -- I hope the analogy is still clear.)
Double-yellow line analogy: You really shouldn't drive in the wrong lane against the flow of traffic because you might get yourself or someone else killed. Of course, there's nothing to stop you except the law, which will prosecute you after the fact if you're caught and can't explain yourself well.
Concrete lane barrier analogy: Even if you had a really, really good reason to drive in the wrong lane, you can't. In fact, if traffic is backed up and you have an emergency in the car, you can't even make an illegal U-turn to go the other way to get back the way you came.
My point is that Perl isn't really about the second way of doing things. And I suspect that many (most?) Perl programmers are sufficiently happy with the first that at most a simple caller() check is all they deem worthwhile. Anyone who really wants around it is going to find a way and there are diminishing (or even negative) returns in the time required to even try to stop them.
Just for fun, if you really want to bend your brain in the "subterfuge race", don't forget about overriding CORE::GLOBAL::caller() or the "# line 43 Bar.pm" trick. (See the very end of perlsyn for the last one.) Even for "private" lexical subs, I bet one could get at them with PadWalker.
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