// was standard in 5.10 and available in 5.8.x with a series of patches, so that one is not the best reason to require 5.12 (instead of 5.10).

Using my version of perlver-fast:

$ perlver-fast -ve '$a //= 10'
-e: 5.010 / 5.010
5.010      //= operator
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Whatever the reasoning behind a minimal requirement is, as long as the argumentation is good enough, it is acceptable.

Dropping support for alder perl releases does not mean that the releases of a module that are available on CPAN (and BackPAN) suddenly stop working.

If raising the minimal supported version of perl (for whatever reason) raises the quality as that increases test coverage, I won't object.

A reason not to raise that bar is if your module is way up-river, and thus also raises the minimal supported perl version to all the modules that require yours. Just something to consider.