use constant PUBLIC => 0, PRIVATE => 1;
%query = (
           action1 => { visibility => PUBLIC,
                        code       => \&handler1, },
           action2 => { visibility => PUBLIC,
                        code       => \&handler2, },
           action3 => { visibility => PRIVATE,
                        code       => \&_do_not_advertise, },
          );

my @public_actions =
  grep $query{$_}->{visibility} == PUBLIC, keys %query;

##</code><code>##

sub register_action {
  my( $action, $subname ) = @_;
  no strict 'refs';
  $query{ $action } = { visibility => ($subname =~ /^_/),
                        code => \&{$subname} };
}