Seems like a whole lot of work for something that can be quite reasonably enforced with one of these guards at the start of the method:

if (caller ne __PACKAGE__) { confess "private method invoked" }
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And while there certainly are ways around that, anybody playing those games deserves whatever they get: they’re breaking the law, and it isn’t worth trying to stop them. You’re just trying to stop accidents not nefarious break-ins, for which this is plenty good enough.

It also requires nothing more recent that v5.0 when Carp was introduced, nor lengthening the toolchain at all. There’s a lot to be said for simplicity.