The Camel Book (4th 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,