print join ' ', UniqueWalkers( $struct );

sub UniqueWalkers {
    my %found;
    
    Data::Walk::walk(
        sub {
            if (/^values$/) {
                foreach ( keys %{ $Data::Walk::container->{$_} } ) {
                    $found{$_}++;
                }
            }
            return;
        },
        @_
    );
    return sort keys %found;
}