foreach $primary( @primary )
{
    print "\t$primary\n";

    foreach $secondary( @secondary )
    {
        print "\t\t$secondary\n";

        foreach $tertiary( @tertiary )
        {
            print "\t\t\t$tertiary\n";

            #%HoH = ( $primary => $secondary => $tertiary);
            #%HoH = {$primary}{$secondary} => $tertiary);
            # 
            $HoH{$primary} -> {$secondary} = [] 
                   unless ref($HoH{$primary} -> {$secondary}) eq 'ARRAY';
            push @{$HoH{$primary} -> {$secondary}},$tertiary;
        }
    }
}