Well, here's the XS routine that's causing the memory fault. Like I said, routine works fine for non-threaded perl.

double
__call( sv_class, sv_path, sv_input )
    SV * sv_class
    SV * sv_path
    SV * sv_input
    CODE:
        char *class = SvPV( sv_class, PL_na );
        char *path = SvPV( sv_path, PL_na );
        double input = SvNV( sv_input );
        int fd;
        door_arg_t arg;
        double output;

        fd = open(path, O_RDWR);

        arg.data_ptr = (char *) &input;
        arg.data_size = sizeof(double);
        arg.desc_ptr = NULL;
        arg.desc_num = 0;
        arg.rbuf = (char *) &output;
        arg.rsize = sizeof(double);

        if ( door_call(fd, &arg) == 0 ) {
            RETVAL = output;
        } else {
            printf("ERRNO: %d\n",errno);
            RETVAL = -1;
        }
    OUTPUT:
        RETVAL
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