os/mac: use perl 5.18 on Catalina

[Bo98]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

It was a nice experiment, but it seems a lot of extra modules are missing: /System/Library/Perl/Extras/5.28.

[BrewTestBot]

Review period skipped due to critical label.