routine(a, b=4,...)
  char * a;
  int b

  PREINIT:
    foo_t * c = NULL;

  CODE:
        if (items > 2 ) {
           if (SvOK(ST(2))) { /* defined */
               if (SvROK(ST(2))) { /* reference */
                   IV tmp = SvIV((SV*)SvRV(ST(2)));
                   c = INT2PTR(foo_t *,tmp);
            } else
                Perl_croak(aTHX_ "c is not a reference");
            }
        }
         ... continuing as before