if($validParams) {
# ...
    print $json_output_encoded;
# ...
}
else {
# ...
    Apache2::RequestUtil->request->custom_response(400,$json_output_en
+coded);
[download]

Is this a CGI or some mod_perl code? If it's a CGI and you haven't set something up to load Apache2::RequestUtil by default, this could be the reason for the error. What happens if you do the same thing (print $json_output_encoded;) in both your if branches?