That is odd. I don't understand why this happens. I have a couple of workarounds which work for me, but may or may not be suitable for whatever you're doing:

• Temporarily binmode STDOUT, ':raw' before cloning it.

• Don't clone STDOUT at all; just take a reference to it with my $fh = *STDOUT{IO}. (In this case fileno($fh) and fileno(\*STDOUT) will report the same number.)

Update: I asked about this in #p5p and Zefram agrees that it seems like a bug, or at least the error message could be better. It's now on RT.