> But installing one was the big issue, as it would always choose the latest release of any module in the dependency chain, including the perl interpreter itself.

You are mistaken. No cpan client is capable of upgrading perl itself. If a cpan module declares a dependency on a perl version later than what you are currently running, the installation simply fails (as what would happen if there is a dependency declaration on a module that is shipped with core and is not upgradable separately, e.g. 'strict' or 'POSIX').