is there a way to append to $responseXML in every case, without having to type it every time?

You may wish to use a construct that's composed of '? :', which avoids the use of 'Switch', but I'm not sure if it's more lazy, as it requires some extra typing:

$responseXML .= 
     ( $_ eq 'sloven' ) ? ( sloven('xml') )
   : ( $_ eq ... )      ? ( ... )
   : ( ... test ... )   ? ( ... then ... )
   :                      ( final 'else' value )
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Without seeing more of what you're trying to do, I'd think you should probably look at using a lookup table. (see super search for more details on lookup tables, if you're not familiar with them)