Replying to myself: Actually, having just re-read perlthrtut, I noticed this one line in the middle:

Note that a shared variable guarantees that if two or more threads try to modify it at the same time, the internal state of the variable will not become corrupted. However, there are no guarantees beyond this, as explained in the next section.

The thing I'm still worried about is whether this applies completely to multi-level struct like my HoHoH, as BrowserUK and I have discussed limitations on this in another thread recently.