The Camel Book (4^th edition, p. 411) is similarly vague:

Many built-in functions may be overridden, although (like knocking holes in your walls) you should do this only occasionally and for good reason.

But this 2001 node by danger:

Re: Overloading Perl's Built-In's

has a list of non-overridable built-in functions, together with the claim that these functions may be identified as “the ones defined to return a negative value in toke.c in the perl source code.” Alas, with my almost-zero knowledge of said perl source code, I can’t find any correlation between the given list and the contents of toke.c. :-(

Anyway, hope that helps,