As far as Perl API is concerned, patch to IO::Socket::SSL has solved the issue, thanks to Gisle & Steffen U.

(Now I won't need to add &Flickr::Upload::new to pass LWP::UserAgent object so that SSL certificate path is set for certificates other than Mozilla::CA.)

For other details, see also (a very short list) ...

• x509: skip certs if in alternative cert chain;
• Perl, LWP "certificate verify failed" with paypal.com (already mentioned above).