When faced with such a situation, you may find that it's helpful to look at what the server returns as a response, by way of headers and status code. For example, since $mech->response is basically HTTP::Response, you can directly look at its status code and response line:

my $resp = $mech->response;
if($resp->is_success) { # or $mech->success
# do successful stuff
} else {
# you have failed to get a proper response, inspect what's left of it
$resp->status_line; 
# alternatively, if you have $resp->is_error, you may want to print ou
+t the error itself:
$resp->error_as_HTML;
}
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