Ah, I should have said that I already have that.

Apache/mod_perl is loading the perl DLL okay. The logic for looking up missing environment variables in the registry is actually contained in that perl DLL, so it is that which picks up the bogus PERL5LIB setting from the registry and then goes off loading the wrong versions of C extension modules' DLLs.

(The extension modules' DLLs then fail to load because they have a dependency (shown by running depends.exe) on the perl56.dll (sic) of the perl installation that resides in the location pointed to by the PERL5LIB registry setting, and that DLL can't be found because it isn't on any PATH and I don't have a LoadFile directive for it. I don't want to load perl56.dll, of course, so fixing that problem wouldn't be the fix. What I want is to stop perl58.dll from loading up the wrong versions of C extension modules' DLLs in the first place.)