Perl5 リファレンス

       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
               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.