site.pp:
$ldap_ssl_enabled    = true

node "admin.internal" {
  $primary_interface = "eth1"

  include ldap::master
}

modules/ldap/manifests/master.pp:
class ldap::master inherits ldap::common {
  case $ldap_ssl_enabled {
    true: { notice("SSL is enabled") }
    default: { notice("SSL is disabled") }
  }

  ... some resources
}

output:
admin:~# puppetd --test
info: Retrieving plugins
notice: Ignoring cache
info: Caching catalog at /var/lib/puppet/state/localconfig.yaml
notice: Starting catalog run
notice: Finished catalog run in 0.22 seconds