give me a complete, foolproof, runnable solution

Sure. My rate is $75 USD / hour, 2 hour minimum.

That is not how perlmonks works. The people here are volunteering their time. To demand a full and complete solution is rather presumptuous.

I have heard that File::Tee will do the job, but that is a link to a module, so you are probably not interested, even though it does look like the documentation even provides a full solution.

File::Tee fails on Windows.

However this is not as simple a problem as I first thought. First I thought you should tie STDOUT to a self defined package. However if you use 'print' in method 'PRINT' it becomes recursive. I have given up on this.

The only solution with which I can come up is to open the program as a pipe from an other script. This is my ad hoc very unpolished solution:

my $what=shift;

use IPC::Open3;

use IO::Handle;

use threads;

unlink('out.txt');

print "Going to run $what.n";

$pid = open3( \*CHILD_IN, \*CHILD_OUT, \*CHILD_ERR, "perl.exe $what" )
+;

autoflush CHILD_OUT;
autoflush CHILD_ERR;

threads->create(\&handle_child_out)->detach;
threads->create(\&handle_child_err)->detach;

while(1){};

sub handle_child_out
{
do
    {
    
    sysread CHILD_OUT, $content, 10;
    if($content ne '')
        {
        
        print "$content";
        
        open OUT,'>>out.txt';
        print OUT $content;
        close OUT;
        
        }
    
    }
    while(1);
}

sub handle_child_err
{
do
    {
    
    sysread CHILD_ERR, $content, 10;
    if($content ne '')
        {
        
        print "$content";
    
        open OUT,'>>out.txt';
        print OUT $content;
        close OUT;
        
        }
    
    }
    while(1);
}
[download]

IO::Tee then.

Jenda
Enoch was right!
Enjoy the last years of Rome.

Add 2>&1 | tee out.txt to your command (or equivalent in the script).

You'll get better results if you use $| = 1; in the script, but it's not necessary to fulfill your requirements.

tail -f out.txt
[download]

I need a solution which prints STDOUT and STDERR both to the screen and to a file at the same time.

use strict;
use Log::Log4perl qw(get_logger);

# Define configuration for file test.log, STDERR, and STDOUT
my $conf = q(
        log4perl.logger                         = ALL, FileApp, Screen
+STDERR, ScreenSTDOUT

        log4perl.appender.FileApp               = Log::Log4perl::Appen
+der::File
        log4perl.appender.FileApp.filename      = test.log
        log4perl.appender.FileApp.layout        = PatternLayout
        log4perl.appender.FileApp.layout.ConversionPattern = %m%n

        log4perl.appender.ScreenSTDERR          = Log::Log4perl::Appen
+der::Screen
        log4perl.appender.ScreenSTDERR.stderr   = 1
        log4perl.appender.ScreenSTDERR.layout   = PatternLayout
        log4perl.appender.FcreenSTDERR.layout.ConversionPattern = %m%n

        log4perl.appender.ScreenSTDOUT          = Log::Log4perl::Appen
+der::Screen
        log4perl.appender.ScreenSTDOUT.stderr   = 0
        log4perl.appender.ScreenSTDOUT.layout   = PatternLayout
        log4perl.appender.ScreenSTDOUT.layout.ConversionPattern = %m%n
);

# Initialize logging behaviour
Log::Log4perl->init( \$conf );

# Obtain a logger instance
my $logger = get_logger("Chess::GUI");
$logger->info("This is going to test.log, STDERR, and STDOUT.");
[download]

Log::Log4perl is quite powerful. You could try experimenting with different conversion patterns as explained in Log::Log4perl::Layout::PatternLayout.