Hi Don. I like your idea and tried your code out a bit. You mentioned in your intro that you think there are better solutions possible; you'll probably like this one, since it doesn't require a separate process:

use strict;
use warnings;
use feature 'say';
use autodie qw( :all );

open my $log, '>', 'output.txt';
tie *BOTH, 'Multiplex', *STDOUT, $log;

select *BOTH;   say 'Sent to display and log : ', 'tested';
                printf "%s should get %s\n", qw( Both this );
select *STDOUT; say 'Sent to display only';
select $log;    say 'Sent to log only';

package Multiplex;
use parent 'Tie::Handle';
sub TIEHANDLE { my $class = shift; bless [ @_ ], $class }
sub PRINT { my ($self,@vals) = @_; print $_ @vals for @$self }
sub PRINTF { my ($self,@vals) = @_; printf $_ @vals for @$self }
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Just before pressing "create" here, I decided to search CPAN. Sure enough there is at least one module [Tie::FileHandle::MultiPlex] that implements almost exactly this idea. But I couldn't get it to work with my printf. I'm sure there are pitfalls with my quick hack at this too.