The first list in CORE gives you keywords that definitely cannot be overridden using use subs. I think perhaps that anything not listed there may be overridden, but I could be wrong.

Generally the best way to find out if a particular function can be overridden is to try. And remember to test it on all the versions of Perl that you care about because this is something that has been changing quite a bit in recent releases - more and more keywords are becoming over-ridable.