1  use strict;
     2  use warnings;
     3  use 5.010;
     4
     5  open my $SAVE_STDOUT, '>&', 'STDOUT';  #<--quoted bareword
     6  open STDOUT, '>', 'data1.txt';
     7  say 'hello world';
     8
     9  open my $SAVE_STDERR, '>&', 'STDERR';  #<--quoted bareword
    10  open STDERR, '>', 'errors.txt';
    11  warn "my warning message";
    12
    13  open STDOUT, '>&', $SAVE_STDOUT;
    14  open STDERR, '>&', $SAVE_STDERR;
    15  say 'goodbye';
    16  warn 'my warning message #2';

--output:--
goodbye
my warning message #2 at 2perl.pl line 16.

$cat data1.txt
hello world

$cat errors.txt
my warning message at 2perl.pl line 11.