static char *
my_sv_unchop(pTHX_ SV *sv, STRLEN size) {
    STRLEN len;
    char *pv = SvPV(sv, len);
    IV off = SvOOK(sv) ? SvIVX(sv) : 0;
    if (!size)
        return pv;

    if (off >= size) {
        SvLEN_set(sv, SvLEN(sv) + size);
        SvCUR_set(sv, len + size);
        SvPV_set(sv, pv - size);
        if (off == size)
            SvFLAGS(sv) &= ~SVf_OOK;
        else
            SvIV_set(sv, off - size);
    }
    else if (len + size <= off + SvLEN(sv)) {
        if (off) {
            SvLEN_set(sv, SvLEN(sv) + off);
            SvFLAGS(sv) &= ~SVf_OOK;
        }
        SvCUR_set(sv, len + size);
        SvPV_set(sv, pv - off);
        Move(pv, pv + size - off, len, char);
    }
    else {
        SV *tmp = sv_2mortal(newSV(len + size));
        STRLEN tmp_len;
        char *tmp_pv;
        SvPOK_on(tmp);
        tmp_pv = SvPV(tmp, tmp_len);
        Move(pv, tmp_pv + size, len, char);
        SvCUR_set(tmp, size + len);
        sv_setsv(sv, tmp);
    }
    return SvPVX(sv);
}