Here is an example. Some commands need IPC::Open3, and you do "my $pid = open3(.....)

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

my $mw = MainWindow->new(-background => 'gray50');

my $text = $mw->Scrolled('Text')->pack();

my $pid;
my $startb = $mw->Button( -text => 'Start',
                -command=> \&work,
                 )->pack();

my $count = 0;
my $label = $mw->Label(-textvariable=>\$count)->pack();

my $testtimer = $mw->repeat(500,
                   sub { $count++}  );

my $stopb = $mw->Button( -text => 'Exit',
                -command=>sub{
                 kill 9,$pid;
                 exit;
              },
                 )->pack();

MainLoop;
#####################################

sub work{

$startb->configure(-state=>'disabled');

use Fcntl;                                                            
+             
  my $flags;                                                          
+               
 
  #long 10 second delay between outputs
  $pid = open (my $fh, "top -b -d 10 |" ) or warn "$!\n";

  fcntl($fh, F_SETFL, O_NONBLOCK) || die "$!\n"; # Set the non-block f
+lags  

   my $repeater; 
   $repeater = $mw->repeat(10,
          sub {
                if(my $bytes = sysread( $fh, my $buf, 1024)){;
            $text->insert('end',$buf);    
                $text->see('end');         
          }
      }
     );             
}
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P.S. Hurry with your questions, I won't be here for long...:-)