|No such thing as a small change|
No, that first block doesn't look like what I described. That first block looks like the XS version is bundled with the interface module and they try to detect if the XS part of the one module just isn't installed. Trying to install a module distribution where you can't build the XS part is ugly. So it is nice of them to try to detect that situation but it doesn't look to me like they've really done anything to make life easy for people who have a need for that situation.
So, splitting out the pure-Perl version into a separate package doesn't bother me one way or the other in the abstract. Splitting it out into a separately distributed module might be a step backward, I agree, though perhaps only a tiny step. I prefer to have the interface distribution be the "visible" one with the pure-Perl implementation less visible but required by (or included in) the interface distribution, since the alternate situation (people having to specifically request that the pure-Perl implementation be installed) tends to lead to less transparent fall-back.
If the XS code were similarly split out and the XS code distribution declared that it depends on the base interface distribution, then that would be a big step forward.
In this particular case, I suspect that part of the motivation for these changes is that this particular module looks like it was added to "core" (based on comments in the "Changes" file) and so the XS component should be built when Perl is built and so most people shouldn't have to worry about trying to build the XS part. Of course, this idea that "/it/ being in core means that everybody who gets Perl also gets /it/" is not absolute.
For example, my primary Linux box has a Perl that is missing many core modules and that box doesn't have a C compiler installed (there at least used to be a C compiler available but it isn't easy to find nor install and I'd probably have to purchase extra hardware to do that anyway). So I don't buy the "who cares if they can't build an XS module, nearly everybody can if they just take the time" argument. Time is valuable so not having to take the time is a valuable option to provide to people. I've seen plenty of cases where time isn't the only obstacle (in my case, hardware restrictions, in many cases company policy restrictions or factors of the scale involved due to a large number of target platforms, etc).
So it'd be nice to help these modules be better citizens toward those unfortunate enough to not have the ideal Perl build environment to work with. But my experience is that even when accomodating such cases is not complex nor difficult, many will still object to having to even contemplate the possibility at all, considering it a waste of their valuable time. So I wish you luck.
But, yes, for this particular case, the creation of MIME::Base64::Perl looks like a backward step to me as well.
It is sad that somebody expects one to do s/MIME::Base64/MIME::Base64::Perl/g to their code base (including to modules one didn't write) in order to allow it to work in a unfortunate environment, especially when this previously wasn't required.