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();
        else {
            char *name = SvPV_nolen(*args);
            if (*mode == IoTYPE_NUMERIC) {
                fd = PerlLIO_open3(name, imode, perm);
                if (fd >= 0)
                    return PerlIO_fdopen(fd, (char *) mode + 1);
            }
            else if (old) {
                return PerlIO_reopen(name, mode, old);
            }
            else {
                return PerlIO_open(name, mode);
            }
        }
    }
    else {
        return PerlIO_fdopen(fd, (char *) mode);
    }
    return NULL;
}

##</code><code>##

...
if ((*type == IoTYPE_RDWR) && /* scary */
 (*(type+1) == IoTYPE_RDONLY || *(type+1) == IoTYPE_WRONLY) &&
 ((!num_svs || (tend > type+1 && tend[-1] != IoTYPE_PIPE)))) {
 TAINT_PROPER("open");
 mode[1] = *type++;
 writing = 1;
 }

##</code><code>##

else if (*type == IoTYPE_RDONLY) {
            /*SUPPRESS 530*/
            for (type++; isSPACE(*type); type++) ;
            mode[0] = 'r';
#ifdef HAS_STRLCAT
            if (in_raw)
                strlcat(mode, "b", PERL_MODE_MAX);
            else if (in_crlf)
                strlcat(mode, "t", PERL_MODE_MAX);
#else
            if (in_raw)
                strcat(mode, "b");
            else if (in_crlf)
                strcat(mode, "t");
#endif
            if (*type == '&') {
                goto duplicity;
            }
            if (*type == IoTYPE_STD && (!type[1] || isSPACE(type[1]) || type[1] == ':')) {

##</code><code>##

...
      else {
                if (!num_svs) {
                    namesv = sv_2mortal(newSVpvn(type,strlen(type)));
                    num_svs = 1;
                    svp = &namesv;
                    type = Nullch;
                }
                fp = PerlIO_openn(aTHX_ type,mode,-1,0,0,NULL,num_svs,svp);
            }

##</code><code>##

            if (!fp && type && *type && *type != ':' && !isIDFIRST(*type))
               goto unknown_open_mode;
        }