Again, it looks like a genuine bug in POSIX builds of Perl or the underlying libraries.

As I don't run any of the systems where the failure occurs, I cannot help you further. There is really nothing you can do about it other than report it to the package mainatainer and/or p5p (via perlbug).

You might be able to work around the bug by:

1. Passing the fileno of the open file handle to each thread explicitly;
2. Use POSIXdup() to clone the file descriptor manually;
3. Use POSIXwrite() to write to it.

In theory, you can use 'dupping' syntax of open (open my $fh, '>&', $fno) to achieve this, but chances are that is essentially what is being done when an open filehandle is automatically cloned, and so it would likely suffer the same problems if you do it yourself.