#!/usr/bin/perl 
use strict;
use warnings;
use POSIX qw(dup2);

open FH, '>', 'log';
dup2(fileno FH, 2);
open STDERR, '>', 'stderr';
# this would be placed into 'stderr', not into 'log'
warn "STDERR descriptor is:", fileno(STDERR), "\n";
die "exit from program\n";