=over

=item prototype FUNCTION

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

=back