I'm almost 100% certain I built that in exactly the same way

Hmmm ... apart from using its git utility on my Windows 7 box, I don't do much with Cygwin.
However, a month or so back I built and installed perl-5.33.4 on Cygwin by configuring with:

sh Configure -des -Dusedevel -Uversiononly -Dprefix=/home/sisyphus/per
+l-5.33.4-d
[download]

I've just now built and installed perl-5.32.0 by configuring with:

sh Configure -des -Dprefix=/home/sisyphus/perl-5.32.0-d
[download]

(The -Dusedevel -Uversiononly switches are specific to devel releases.)
The config.h files produced by both builds do not define I_XLOCALE.

So I'm still not sure how to account for the issue you faced.
Not that it matters very much - the important thing being that it has apparently been fixed !!

Cheers,
Rob