Thanks to both of you for these clarifications. It sounds like this will be the only significant hurdle and that it can be overcome (or at least dodged) by out of band means. One step is to maintain a page elsewhere with clear and easy access to the lastest legacy version. In tandem with this, the docs for the newer version should point to that page and ideally the installation process should check the running version of perl and error out if it is too old with a suitable message to point to the legacy version.
For now, this seems like the best approach. It should give the legacy users enough pointers to their newest version and allow me to develop and release both branches independently.
Thanks everyone for your very useful comments.