int
Perl_magic_getsubstr(pTHX_ SV *sv, MAGIC *mg)
{
    STRLEN len;
    SV * const lsv = LvTARG(sv);
    const char * const tmps = SvPV_const(lsv,len);
    I32 offs = LvTARGOFF(sv);
    I32 rem = LvTARGLEN(sv);
    ...
}

int
Perl_magic_setsubstr(pTHX_ SV *sv, MAGIC *mg)
{
    dVAR;
    STRLEN len;
    const char * const tmps = SvPV_const(sv, len);
    SV * const lsv = LvTARG(sv);
    I32 lvoff = LvTARGOFF(sv);
    I32 lvlen = LvTARGLEN(sv);
    ...
}