prototype FUNCTION
               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 if its 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.