Rakudo-Star is getting pulled into your build by PPM-Repositories. You need to remove PPM-Repositories. ActivePerl-PPM and PPM-Repositories are incompatible with 5.28, and with being built on The Platform.
Select the trash can icon beside PPM-Repositories, remove it, and Commit the change.
That will clear up the initial problem. In the meantime we have some new issues with other common Perl modules, so watch this post and remove PPM-Repositories after we have determined why the other modules are failing.