use Inline C => q[
void avar_cee(SV *sv1, SV *sv2)
{
    char *sv1p, *sv2p;
    STRLEN sv1len, sv2len;
    SV *sv1_sv;
    STRLEN i;

    if (!SvROK(sv1) || !SvPOK((SV*)SvRV(sv1)) || !SvPOK(sv2)) {
        croak("Usage: avar_cee(\$s1, $s2)");
    }

    sv1_sv = (SV*)SvRV(sv1);
    sv1p = SvPV(sv1_sv, sv1len);
    sv2p = SvPV(sv2,    sv2len);

    if (sv1len != sv2len) {
       croak("The given strings must be of the same length");
    }

    for (i = 0; i < sv2len; i++) {
        if (sv1p[i] == '\0') {
            sv1p[i] = sv2p[i];
        }
    }
}