prototype FUNCTION
    prototype
            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.