$box{$X}{$Y}{$z} =  1;             # DOES work 
#      $box{$X}{$Y}{$z}{'rect'} = 1 ;     # 'works' because it's commented out
#      $box{$X}{$Y}{$z}{'rect'} = undef ; # see above
       $box{$X}{$Y}{$z}{'text'} = 2;      # refs error