オーバーライドできない関数

Perl Cookbook 2ndより。

Not all reserved words have the same status.
Those that return a negative number in the C-language keyword( ) function in the toke.c file
in your Perl source kit may be overridden.
Keywords that cannot be overridden as of v5.8.1 are
defined, delete, do, else, elsif, eval, exists, for, foreach,
format, glob, goto, grep, if, last, local, m, map, my, next,
no, our, package, pos, print, printf, prototype, q, qq, qr, qw,
qx, redo, require, return, s, scalar, sort, split, study,
sub, tie, tied, tr, undef, unless, untie, until, use, while, and y.
The rest can.

あとperldoc perlsubにbuilt-inがどーたらと書いてある。