(I guess that's just a homespun, poor person's Perlbrew ;-)

Sounds like it :-)

For example, what happens when you then run cpan -i Some::Module and the first perl in the path doesn't have a 'cpan' utility (because it's named 'cpan5.31.7') but the next perl in the path does have a utility named 'cpan' ?

Yes, that's a good point, that's what I meant when I wrote to check which perl vs. which cpanm. It's also why I recommended cpanm, because as long as you point its installation (curl -L https://cpanmin.us | perl - App::cpanminus) to the correct perl binary, it should get installed in the correct bin directory.

I'm not sure that, even if Perlbrew had built these perls, it could handle the required switching of compilers ?? Come to think about it, I don't actually know if Perlbrew can be used to build perl on Windows at all.

No, but stevieb's berrybrew can. Each distro of Strawberry Perl comes with a compiler, and berrybrew also switches the PATH entries for those as well.