Note that I'm addressing the Unix side of the issue only

Oops, for some reason, I got the impression the thread was about Windows.

I can still see one issue on Unix when passing '-' as the command. In that case, the FOO handler (such that fileno(FOO) == 1) may be being used on the child Perl code. Though, this can be worked around easily just checking that the command is not '-'.

Anyway, if you just want a Unix solution, why don't to use POSIX::dup2?.