Hi bliako,
Thanks for taking the time to answer me. I tried your suggestion splitting the host and API path from the request and I also put the '&format=json' into each request string before calling post, so the "&token=$token" is then put last in the edit request.

I'm afraid the result is a step back. Now I get an API help page in HTML from the first post. This I get at the first log attempt to get the token, which worked fine before. I updated the post sub like this

sub post {
    my $query = shift @_;
    # my $req = "$api?$query"; # This removed
    print "Fetching data with: POST $api $query\n" if $debug;
    print Dumper $client;
    $client->POST($api, $query);

    if ( $client->responseCode() == 200 ) {
        print "Got Data OK, decoding ... " if $debug;
        print Dumper $client->responseContent() ;
        my $data = $JSON->decode( $client->responseContent() );
        print Dumper $data;
        return $data;
    }
    else {
        print "Failed with code $client->responseCode()\n";
        print Dumper $client->responseContent();
    }
}
[download]

Cheers,
R.