How to reference a hiera variable from elsewhere the hierarchy?

You can use Hiera lookup functions within Hiera. Documentation here: https://docs.puppetlabs.com/hiera/1/variables.html#using-lookup-functions

In your case you would use:

server_name: "service-%{hiera('env_name')}.%{::domain}"

This is a priority lookup and will get the value for the key env_name from the highest (most specific) hierarchy level in which it finds the key.

This requires Hiera 1.3 or higher.

For Hiera 5, used in Puppet 5, use the following syntax:

server_name: "service-%{lookup('env_name')}.%{::domain}"

...where %{lookup('env_name')} is the part that gets the data from other hiera key.

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