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 a string starting with CORE::, the rest is taken as a
name for Perl builtin. If the builtin is not overridable (such as
qw//) or its arguments cannot be expressed by a prototype (such as
system) returns undef because the builtin does not really behave
like a Perl function. Otherwise, the string describing the equivalent
prototype is returned.