Which Perl built-ins cannot be overridden in CORE::GLOBAL?

The Overriding Built-in Functions section of the perlsub documentation provides

There is a second method that is sometimes applicable when you wish to override a built-in everywhere, without regard to namespace boundaries. This is achieved by importing a sub into the special namespace CORE::GLOBAL::.

and then gives a few examples. At the end, however, is

Finally, some built-ins (e.g. exists or grep) can't be overridden.

What is the full list?