My best guess is that %Config is some kind of tied or magic-applied hash and it doesn't like having its elements aliased

Yes, in order for MinGW to work with ActivePerl, it's necessary to override a number of %Config values. They do this with something along the lines of:

my $obj = tied %Config::Config;
if(mingw_detected) {
  $obj->{cc} = 'gcc';
  $obj->{make} = 'dmake';
  ....
}
[download]

I still don't get how the presence (and loading) of an empty blib can make a difference to the loading of Devel::CheckLib ... though it's probably not as important as I first thought.

I'll see how I go with what you've come up with already. Thanks !

Cheers,
Rob