However, I posted the $response->content; held in variable $json_text. (I thought this is the most important part to understand why decode_json fires an error.

As Corion and I already hinted at, it's pretty unlikely that you posted exactly what $json_text contains. Please use a module such as Data::Dumper or Data::Dump.

To output a CGI header with a different Content-Type, you can say print $q->header("application/json");