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 builtin is not overridable (such as qw//) or its arguments cannot be expressed by a prototype (such as system()) - in other words, the builtin does not behave like a Perl function - returns undef. Otherwise, the string describing the equivalent prototype is returned.