You're doing weird things with the low-level parts of the LWP API. This is much shorter and actually working as intended (takes care of assembling and encoding the request the correct way):

use LWP::UserAgent qw();
my $response = LWP::UserAgent
    ->new
    ->post(
        $buildManagerUrl,
        {
            action => $action,
            params => $callParamsJson,
        }
    );
[download]