You need to use fileevent with Tk to read filehandles. (On win32, fileevent only works on sockets). There are many other complexities that can set in, but this is the basic.

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

$|=1;
open( T,"top -d 1 -b  & |");

my $mw = new MainWindow;

my $t = $mw->Scrolled("Text",-width => 80, -height => 25, -wrap => 'no
+ne');
                                             $t->pack(-expand => 1);

$mw->fileevent(\*T, 'readable', [\&fill_text_widget,$t]);
MainLoop;
       
sub fill_text_widget {
          my($widget) = @_;
          my $text = <T>;
      $widget->insert('end',$text);
          $widget->yview('end');
}
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