on Unix, pipeing the output to a new forked process is the only way that would work in any case:

select STDERR;
$|=1

unless (open STDERR, '|-') {
  eval {
    open ERRLOG, ">/tmp/errlog" or die;
    while(<>) {
      print ERRLOG $_;
      print STDERR $_;
    }
  };
  exit(0);
}

# repeat for STDOUT
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