the code was untested.

I don't have time to look at this just now, but it's surely a problem with socketpair's args.

Start by changing

        _pipe(*TO_CHLD_R,     *TO_CHLD_W    ) or die $!;
        _pipe(*FR_CHLD_R,     *FR_CHLD_W    ) or die $!;
        _pipe(*FR_CHLD_ERR_R, *FR_CHLD_ERR_W) or die $!;
[download]

to

        _pipe(*TO_CHLD_R,     *TO_CHLD_W    ) or die $^E;
        _pipe(*FR_CHLD_R,     *FR_CHLD_W    ) or die $^E;
        _pipe(*FR_CHLD_ERR_R, *FR_CHLD_ERR_W) or die $^E;
[download]

to get a better error message.