I had to do something similar on my Tie::Array::Packed module:

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);
}
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this function inserts size bytes in front of the string efficiently and returns a pointer to the start of the new pv.

You would probably want to modify it so that more bytes than requested are reserved on the string to optimize consecutive insertions.

Note that sv_setsv is used to only copy the string contents once as it steals the SV string memory from the source SV when it is a mortal.

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