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;
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If that's too verbose, just make subs which do the same thing (e.g. register_private( action3 => \&_do_not_advertise )). Or encapsulate things in a class and make visibility a method.

Update: Or just have a register sub which takes an action name and a sub name and then adds the right elements.

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