Returns the prototype of a function as a string (or undef if the
function has no prototype). FUNCTION is a reference to, or the name of,
the function whose prototype you want to retrieve. If FUNCTION is omitted,
$_ is used.
If FUNCTION is a string starting with CORE::
, the rest is taken as a
name for a Perl builtin. If the builtin's arguments
cannot be adequately expressed by a prototype
(such as system), prototype() returns undef, because the builtin
does not really behave like a Perl function. Otherwise, the string
describing the equivalent prototype is returned.