Thanks demerphq. I vaguely remembered reading an explanation of this a long time ago, but not where, nor any of the details. That explains it nicely.

When I saw the "won't stay shared" warnings, saw that several of them related to arrays, and then looked inside and saw that several of the subroutines in question are being used within or from Tk UI callbacks, which tend to gets cycles many times, the alarm bells rang.

It wasn't possible to actual do anything useful with the program to verify the notion, as it requires soecial format files.