I think you should:
$sz = fpathconf(fileno($wd), _PC_PIPE_BUF)
        or die "fpathconf(pipe): $!\n";

[download]
From what I see in POSIX.xs, the conversion of a Perl file handle to a file descriptor is not done for you.

In reply to Re: Why does fpathconf give "bad file descriptor"? by ReturnOfThelonious
in thread Why does fpathconf give "bad file descriptor"? by madscientist

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