[Puppet Users] Accessing hiera_hash values in puppet templates

I am setting up a nagios hosts definition file using ec2 data with puppet
3.2.

The hiera JSON file generated looks like this:
  {
     "instances": {
         "prod-api-01": {
             "private_ip": "10.1.1.1",
             "public_ip": "23.1.1.1",
         },
         "prod-api-02": {
             "private_ip": "10.2.2.2",
             "public_ip": "23.2.2.2",
         },
         "prod-api-03": {
             "private_ip": "10.3.3.3",
             "public_ip": "23.3.3.3",
         }
     }
}


My manifest has the following declarations:

$instances = hiera_hash('instances')
$instance_names = keys($instances)


In my hosts.cfg.erb template, I attempted the following, but neither works
(entries for alias and address are blank).

<%- @instances.each_pair do |instance_name, attributes| -%>
define host {
     host_name <%= @instance_name %>
     use host-platform-prod-group
     alias <%= attributes['public_ip'] %>
     address <%= attributes['private_ip'] %>
}
<%- end -%>

AND

<%- @instance_names.each do | instance_name | -%>
define host {
     host_name <%= @instance_name %>
     alias <%= @instances[@instance_name]['public_ip'] %>
     address <%= @instances[@instance_name]['private_ip'] %>
}
<%- end -%>


I know I can access @instances and @instance_names within the template, but
the template does not appear to find values for the loop variable
@instance_name. Why is this?