This is a rough solution. Note that this works on Windows. You have to find out how to kill a process on your particular system. Stars cmd.exe (in your system replace cmd.exe with the desired shell command), waits for its output to exceed 40 bytes then prints the output, kills cmd.exe, waits for user input, then quits.

use IPC::Open3;
use IO::Handle;
use threads;
use Win32::Process::Kill;

$process='cmd.exe';

$pid = open3( \*CHILD_IN, \*CHILD_OUT, \*CHILD_ERR, $process );

autoflush CHILD_OUT;
autoflush CHILD_ERR;

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

unlink('out.txt');

do
    {
    
    open IN,'out.txt';
    my $content=join('',<IN>);
    my $l=length($content);
    print "Number of bytes read: $l\n";
    if($l>40)
        {
        print $content;
        Kill($pid);
        print "Ready.\n";
        $x=<>;
        exit;
        }
        
    sleep(1);
    
    } while(1);


sub handle_child_out
{
do
    {
    
    sysread CHILD_OUT, $content, 4092;
    if($content ne '')
        {
        
        open OUT,'>>out.txt';
        print OUT $content;
        close OUT;
        
        }
    
    }
    while(1);
}

sub handle_child_err
{
do
    {
    
    sysread CHILD_ERR, $content, 4092;
    if($content ne '')
        {
        
        open OUT,'>>out.txt';
        print OUT $content;
        close OUT;
        
        }
    
    }
    while(1);
}
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