Thanks for this extra info - I now understand that it was only the module versions which were incorrect but the dist version was correct. That explains why I couldn't find version 999.999 on CPAN or BackPAN.

However once any version with the extra $VERSION declaration is installed on a cpan-testers machine, the test machinery (or maybe cpanm) thinks it has version 999.999 and so won't update it.

That should only be temporary, I would have thought. See this recent discussion regarding common dependencies not being present on smoke testers' machines. They might not all work this same way, of course - I'm not running a smoke tester (yet) so can't comment further on that. Anyway, you've taken the plunge with the 4-digit major version now so this is all moot.