Found in /usr/lib/perl5/5.14.1/pod/perlfaq5.pod
   How do I dup() a filehandle in Perl?
   If you check "open" in perlfunc, you'll see that several of the ways to
       call open() should do the trick. For example:

               open my $log, '>>', '/foo/logfile';
               open STDERR, '>&LOG';

       Or even with a literal numeric descriptor:

               my $fd = $ENV{MHCONTEXTFD};
               open $mhcontext, "<&=$fd";  # like fdopen(3S)

       Note that "<&STDIN" makes a copy, but "<&=STDIN" makes an alias. That
       means if you close an aliased handle, all aliases become inaccessible.
       This is not true with a copied one.

       Error checking, as always, has been left as an exercise for the reader.