hook_permission() is the same as in D7

hook_permission() in Drupal 8 has not changed and can be implemented in the .module file as in D7.

function optimizely_permission() {
  return array(
    'administer optimizely' => array(
      'title' => t('Administer Optimizely module'),
      'description' =>
        t('Administer access to everything in module'),

      'restrict access' => TRUE,
    ),
  );
}

What's different is that the permissions defined in hook_permission() are then used in the  .routing.yml  file to control access to specific paths. For instance,

optimizely.settings:
  path: /admin/config/system/optimizely
  # ...
  requirements:
    _permission: administer optimizely

Update for Drupal 8, beta 1: hook_permission() has been removed. Its functionality is now provided by the .permissions.yml file. See http://optimizely-to-drupal-8.blogspot.com/2014/11/hookpermission-has-disappeared.html


Source:

DRUPAL 8 INFO HOOK TO PLUGIN, DrupalCon Prague, Sept 2013
https://groups.drupal.org/files/pwolanin-2013-09-info-hook-plugin-d.pdf

This presentation states: "Info hooks that simply return a data array - like hook_permission() - without associated functionality - are not candidates to become plugins." I take this to mean that these kinds of hooks will largely remain unchanged.