use Filter::Handle qw/subs/;
## $LOG_FH is a FileHandle reference...
Filter \*STDERR, 
  sub { local $_ = @_;
        if (defined $LOG_FH) {print $LOG_FH "STDERR: $_";}
        $_ 
  };

## Need a signal handler to capture warnings from carp 
## and warn which are not captured by Filter::Handle 
## (but by capturing ourselves and printing to STDERR, 
## they do get picked up by Filter::Handle)...

$SIG{__WARN__} = sub
                 {    local $_ = "@_";
                      print STDERR $_;
                 };