Cannot install hiera by brew
Short answer

Please try brew cask install puppet-agent. Hiera is included.

More details

I tried installing the https://downloads.puppetlabs.com/mac/hiera-1.3.4.dmg file that the cask recipe install manually and got the following warning:


  This package is incompatible with this version of OS X and may fail to >install.
  
  Installing this package may damage your system, and the installation may >fail. To stop the program and the installation, click “Cancel >Installation”.


I did some googling and found that 1.3.4 is very old.

from: https://tickets.puppetlabs.com/browse/HI-533


  Hiera 1.3.4 is old (> 2 years). Much older than OSx 10.11.6. There are >later releases of hiera,"


from: https://docs.puppet.com/hiera/3.1/


  To get started with Hiera, you’ll need to do all of the following:
  
  Make sure it’s installed. Hiera ships with Puppet agent, so install the >puppet-agent package if you haven’t already. (Linux, Windows, OS X.)