Another solution could be to lock the keys of $config after its been configured:

  use Hash::Util 'lock_keys';

  our $config;
  $config->{file_paht} = "/opt/app/data_file";
  lock_keys( %$config );

Now any attempt to use an undefined key will throw an error. So on the open() it would now throw error: Attempt to access disallowed key 'file_path' in a restricted hash at...