GV *to_string = gv_fetchmethod (SvSTASH (sv), "toString");
if (to_string) {
    dSP;
    SV *res_sv; /* result scalar */

    ENTER;
    SAVETMPS;

    /* Declare function arguments */
    PUSHMARK (SP);
    XPUSHs (sv_bless (sv_2mortal (newRV_inc (sv)), SvSTASH (sv)));
    PUTBACK;

    /* Call in scalar context (G_SCALAR) */
    call_sv ((SV *)GvCV (to_string), G_SCALAR);
    SPAGAIN;

    /* Fetch returned SV from the stack */
    res_sv = POPs;
    PUTBACK;

    /* Append returned SvPVX to our SV */
    append_sv(res_sv, result);

    FREETMPS;
    LEAVE;
}

##</code><code>##

void
func (SV *scalar)
    PPCODE:
        SV *result = newSV(INIT_SIZE);
        sv_setpv(result, "");
        SvPOK_only(result);
        serialize(result, scalar);
        XPUSHs(result);

##</code><code>##

SV*
func (SV *scalar)
    CODE:
        SV *result = newSV(INIT_SIZE);
        sv_setpv(result, "");
        SvPOK_only(result);
        serialize(result, scalar);
        RETVAL = result;
    OUTPUT:
        RETVAL