In theory, letting these be set once wasn't an awful idea because it allows
users choice on where Perl modules end up.

In reality, as we've moved closer to sandboxing and at the same time accepted more
perl binding options into formulae it has been exposed as something of a hellraiser.

It's not that uncommon for Perl users to set a custom `INSTALL_BASE` in the ENV which
means we end up with quite a few formula that fail hard with this sort of error:

```
Only one of PREFIX or INSTALL_BASE can be given. Not both.
```

The other common error, which we discovered via sandboxing, is that the Perl modules
end up outside Homebrew's control which means we don't automatically remove them
when the package is removed. Sandboxing blocks this, which means when we eventually
move to enable that by default for users as well as CI it'll quickly become a
prominent issue.