is_deeply(scalar $pdf->adjust_to_bounding_box($main_image),  
          undef, 
          "adjust_to_bounding_box returns on no second param" 
         ); # test

##</code><code>##

is_deeply([$pdf->adjust_to_bounding_box($main_image)],  
          [], 
          "adjust_to_bounding_box returns on no second param (list context)" 
         ); # test