Hi, it's most common nowadays to pass JSON around in requests and responses as the entire body (see, e.g., almost any API you use). Try this:

#use HTTP::Request::Common;
use HTTP::Request;

my $request = HTTP::Request->new(POST => $url);
$request->header( 'Content-Type' => 'application/json' );
$request->content( $json );
...
[download]

... and ...

my $json = $query->param('POSTDATA');
my $data = decode_json( $json );
...
[download]

(see https://metacpan.org/pod/release/LDS/CGI.pm-3.43/CGI.pm#HANDLING_NON-URLENCODED_ARGUMENTS)

Hope this helps!