If indeed a missing module prevents CPAN.pm from using its distroprefs, what is a minimal set of non-core modules that need to be installed in order for distroprefs to be working?

It turns out that YAML alone makes the difference. Installing this makes *.yml files in distroprefs usable.

As an alternative, the CPAN documentation describes a way to convert YAML files to Data::Dumper format, which should work out of the box. I'd be able to patch YAML or its dependencies via distroprefs that way.

To make expect rules work, too, Expect should come next, I guess. Fortunately, distributions needing expect rules are getting rarer these days.