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
undef, because the builtin does not really behave like a Perl function. Otherwise, the string describing the equivalent prototype is returned.