In your server code you have the following line:

print "Content-type: application/json;\n\n";
[download]

What is that line doing there and are you certain that the rest of the script hasn't already sent complete headers? This would easily explain why $response->decoded_content starts with Content-type:.

Update: And indeed, in your server code, you have print $q->header;. Printing more headers after that will not add those lines to the headers but to the reply body. See the ->content_type method of CGI.

This also makes your diagnosis of

My response $json_text (an array with a list of URI) looks like this:

somewhat suspect because there must (or rather, should) be a line Content-type: application/json; right before that.