#!/usr/bin/perl
use strict;
use warnings;

open my $oldout, '>&', STDOUT or die "Can't dup STDOUT: $!";
my $stream;
close STDOUT;
open STDOUT, '>', \$stream or die "Can't open STDOUT: $!";

print "Hello!\n";
$stream .= "This line is new!\n";
print $oldout $stream;