while (1) {
        // Actually allocates at least 256.
        SV* line_sv = newSV(255);

        // We know line_sv has a string buffer, so we can use SvPVX.
        char* line = SvPVX(line_sv);

        if (!fgets(line, 256, fh)) {
            break;
        }

        SvCUR_set(line_sv, strlen(line));
        SvPOK_on(line_sv);

        ...

        // The callback may have changed line_sv,
        // so line might no longer be valid.
        line = SvPVbyte_nolen(line_sv);

        printf("c orig: %sc new: %s", line, modified_line);

        ...
    }