hiera.yaml

---
:backends:
  - yaml
:hierarchy:
  - %{::environment}
  - common

:yaml:
  :datadir: /etc/puppet/hieradata

:merge_behavior: deeper

init.pp

class foo {
  $admin_user = hiera("admin_user")
  file { "/var/foo.conf": 
    ensure  => present,
    content => template("foo/template.erb"),
  }
}