PerlIO *
PerlIO_openn(pTHX_ const char *layers, const char *mode, int fd,
             int imode, int perm, PerlIO *old, int narg, SV **args)
{
    if (narg) {
        if (narg > 1) {
            Perl_croak(aTHX_ "More than one argument to open");
        }
        if (*args == &PL_sv_undef)
            return PerlIO_tmpfile();
...