int call_perl_sub(SV *routine, SV *data_hash)
{
    dSP; /* Get the stack pointer. */

    /* Allocate the temporary memory location. */
    ENTER;
    SAVETMPS;

    PUSHMARK(SP); /* Get the temporary stack pointer. */

    XPUSHs(sv_2mortal(data_hash)); /* Push the hash onto the stack. */

    PUTBACK;

    /* Call the PERL subroutine, ignoring its return value. */
    call_sv(routine, G_DISCARD);

    /* Free the allocated memory. */
    FREETMPS;
    LEAVE;
} /* End of function: call_perl_sub() */

MODULE = TestXS       PACKAGE = TestXS

void callback_sub(file_list, callback)
    SV *file_list;
    SV *callback;
CODE:
    HV *inp_file_list = (HV *) SvRV(file_list);
    HE *next_entry      = NULL;

    /* Initialize the iterator. */
    hv_iterinit(inp_file_list);

    for (next_entry = hv_iternext(inp_file_list);
         next_entry != NULL;
         next_entry = hv_iternext(inp_file_list))
    {
        I32 keylen = 0;
        SV *entry_val = hv_iterval(inp_file_list, next_entry);
        char *filepath = hv_iterkey(next_entry, &keylen);

        sv_setpv(entry_val, "Marked.");

        fprintf(stderr,
                "[UPDATED] %s ==>> %s\n",
                filepath, SvPV_nolen(entry_val));

        call_perl_sub(callback, SvREFCNT_inc(file_list));
    }