int len;
    char* pBeg= SvPV(...,len);
    char* pSrc= pBeg + len;
    char* pDst= SvPV(....) + len;
    while(  pBeg <= pSrc  ) {
        if(  ! *pDst  ) {
            *pDst= *pSrc;
        }
        --pSrc;
        --pDst;
    }