If it is defined in the XS library, it will be available just the same as if it were defined in Perl space. I'm not sure what exactly you tested, and whether you get errors now, and what errors you get when using strict. It seems that WWW::Curl::Easy does something fancy with AUTLOAD, so you might want to inspect %WWW::Curl:: for interesting constant names.

A helpful node title would include "WWW::Curl", btw, and describe the problem, instead of asking a general question that could just as well apply to growing tomatoes on the moon.