For the time being, you will not be able to build a Custom Perl runtime for MacOS. We’re still working on providing MacOS support and will make an announcement and update our UI as soon as it’s available. As of now, we have no firm estimate on when that will happen but it’s on the product roadmap.
For the time being, you’ll have to use one of our pre-built, non-editable Perl runtimes that are available on the Featured Projects page here: https://platform.activestate.com/featured-projects