in reply to 2021 is the year to drop support for Perl < 5.12?
It's all very subjective, of course. Of the four bonuses you've mentioned for use v5.12, the only one I would care about is the defined-or which I do find useful.
For 5.14, the only one there which I would care about is s///r but I use that so infrequently that it hardly matters. The big win for me with 5.14 is m//a which is a godsend when having to deal with the moving target that is the built-in character classes.
I don't really code for 5.6 these days beyond a couple of long-established modules where there is no reason to break it. The defined-or is the thing which most stops me from supporting 5.8 in new code.
So I am trying to transition from using Travis-CI for some of my Perl projects (and no CI at all for the rest) to using Github Actions for all of my Perl projects.
This is actually the point which interests me most. Why are you moving away from Travis?
I have been in the process of moving everything out of Github since the disaster. As even the stable releases would need an update if/when rt.cpan.org goes that will be the kick to move all the remaining stuff out too.
🦛