Update: Please see Initial Devel::UncheckedOps, a macro for perl for notes on how print() can be overridden.

Its a function of toke.c's Perl_keywords() function. The keywords with positive values aren't overridable (so says pp.c's pp_prototype function). I get the impression from S_scan_inputsymbol that glob() and readline()'s overrideability is more a function of explicit support than something handled by the prototype. It also got the impression it might have something to do with print's status as an indirect object method expecting a list. Some guesses anyway.

Maybe this generalizes to: all functions overridable by CORE::GLOBAL:: have must prototypes or be explicitly supported by the perl parser (and the only known exceptions are glob() and readline()).

Added: For those that care, I pulled the list of keywords from toke.c:Perl_keywords() that are marked as not being overridable. You'll notice glob() and readmore() are in this list - again, it works only because of explicit support.

__DATA__
__END__
AUTOLOAD
BEGIN
CHECK
DESTROY
do
delete
defined
END
else
eval
elsif
exists
for
format
foreach
grep
goto
glob
INIT
if
last
local
m
my
map
next
no
our
pos
print
printf
package
prototype
q
qr
qq
qw
qx
redo
return
require
s
scalar
sort
split
study
sub
tr
tie
tied
use
undef
until
untie
unless
while
y
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