The message you're getting (at character offset 0 (before "Content-type: applic...")) makes it sound like there's something wrong with $response->content, but you haven't shown what module you're using to decode the JSON, what $json_text actually contains (using e.g. Data::Dumper or Data::Dump), or what URL you're fetching it from. What you have shown works fine for me. Please provide a runnable SSCCE that we can use to reproduce the error.

use warnings;
use strict;
use Data::Dump;
use JSON::MaybeXS qw/decode_json/;

my $json_text = do { local $/; <DATA> };
dd decode_json($json_text);

__DATA__
{"myData":["https://www.sante.de/fileadmin/user_upload/Stellenanzeige_
+Mitarbeiter_Research__Development.pdf","https://www.national-bank.de/
+fileadmin/user_upload/nationalbank/Vermoegensmanagement/Research/Kapi
+talmarkt_Studien/Kapitalmarktmonitor_Dezember_2018.pdf","https://www.
+acxit.com/en/wp-content/uploads/2018/12/CAREERS_Stellenanzeige_WS_Res
+earch_Deal_Generation_20181211.pdf","https://www.s-a.uni-muenchen.de/
+studierende/jobboerse/jobangebote/aktuelle/20181221_44.pdf","https://
+www.dbresearch.de/PROD/RPS_DE-PROD/PROD0000000000459596/Deutscher_H%C
+3%A4user-_und_Wohnungsmarkt_2018.pdf","https://www.bayernlb.de/intern
+et/media/de/ir/downloads_1/bayernlb_research/multiasset_produkte/Pers
+pektiven.pdf","https://www.borders-in-motion.de/documents/12991/34723
+4/Flyer_Research_Factory_WS18-19.pdf/2baa6987-219a-4cbb-8ab0-da36754f
+1282","https://deka.de/site/dekade_privatkunden_site/get/documents/de
+kade/Publikationen/2018/Volkswirtschaft/Aktuell/20181220_VA_USA_Zinse
+ntscheid.pdf","https://www.dmtm.com/medienverwaltung/medien/181218_Re
+search_Engineer_Website.pdf","https://www.ewi.research-scenarios.de/c
+ms/wp-content/uploads/2018/08/Czock_CV_de_20181214.pdf"]}
[download]

Update: I see Corion made many of the same points.