The variable names in global.yaml are being expanded when the template is expanded. Not sure where I need to escape something

gistfile1.txt

#global.yaml
'puppet::hiera::backends': - file
                           - yaml

'puppet::hiera::logger': console

'puppet::hiera::hierarchy': - '%{::environment}/fqdn/%{::fqdn}'
                            - 'fqdn/%{::fqdn}'
                            - '%{::environment}'
                            - global
'puppet::hiera::yaml':
  :datadir: /var/lib/hiera

#hiera.pp
class puppet::hiera (
  $backends  = ['yaml'],
  $logger    = ['console'],
  $hierarchy = ['global'],
  $yaml      = {':datadir' => '/var/lib/hiera'},
  $other     = {}
) {

#Puppet configuration
  file{'/etc/puppet/hiera.yaml':
    ensure  => present,
    mode    => '0660',
    owner   => 'puppet',
    content => template('puppet/hiera.yaml.erb'),
    require => Package['puppet'],
    notify  => $::puppet::to_notify,
  }

}

#hiera.yaml.erb
<%
  require 'yaml'

  data = {:backends  => @backends,
          :logger    => @logger,
          :hierarchy => @hierarchy,
          :yaml      => @yaml}.merge( @other.class == Hash ? @other : {})
-%>

<%= data.to_yaml%>

#evaluates to
--- 
  !ruby/sym hierarchy: 
    - production/fqdn/rainbowdash.sagenite.net
    - fqdn/rainbowdash.sagenite.net
    - production
    - global
  !ruby/sym logger: console
  !ruby/sym yaml: 
    !ruby/sym datadir: /var/lib/hiera
  !ruby/sym backends: 
    - file
    - yaml