test.xs

Perl_ppaddr_t orig_subhandler;

static OP* my_subhandler(pTHX) {
    OP* out = orig_subhandler(aTHX);

    char *file = CopFILE(cCOPx(out));
    printf("xxx %s\n", file);

    return out;
}

[...]

BOOT:
    {   
        MY_CXT_INIT;
        orig_subhandler = PL_ppaddr[OP_ENTERSUB];
        PL_ppaddr[OP_ENTERSUB] = my_subhandler;
    }