Protect fatpacked perlbrew from existing INC

I found I had to modify my perlbrew to add this at the top so that I only had the system libraries (you could add vendor, too, I suppose).

```
BEGIN {
    use Config;
    @INC = @Config{qw(privlibexp archlibexp sitelibexp sitearchexp)};
}
```

Otherwise switching to/from a non-threaded perl to a threaded perl would break horribly with errors like this:

```
dyld: lazy symbol binding failed: Symbol not found: _Perl_xs_apiversion_bootcheck
  Referenced from: /Users/david/.perlbrew/libs/18.2t@std/lib/perl5/darwin-thread-multi-2level/auto/List/Util/Util.bundle
  Expected in: dynamic lookup
```