You can fix that in one of two ways. Either use use (which is the simplest) or write instead WWW::Curl::CURLOPT_HEADER() and WWW::Curl::Easy::CURLOPT_TIMEOUT() to disambiguate them from barewords to function calls.

Because use performs importing at compilation time, the parser knows that those functions are functions even without the parentheses. If you can't import those functions at compilation time, you'll have to disambiguate yourself or disable strict subs. (Don't disable strict subs.)