From the Perl & LWP (O'Reilly) book:
Check the status of the response:

     $response->is_success()
     $response->is_error()
     $response->is_redirect()
     $response->is_info() (probably not used much...)
[download]

And if it is not a success it has some nice functions to figure out what the problem is:

    $response->code() (numeric code returned, error or not)
    $response->message(); (text status code)
    $response->status_line() (both numeric and string error codes)
[download]

Or as above you can get error_as_HTML and use that.