Two solutions - 1.) use Tk::fileevent, 2.) call the Tk event loop yourself.

Ad 1:

$start-> configure (-text => "Start", -command => [\&count]);
MainLoop();

{
    my $handle;
    sub count {
        if ($handle) { # get rid of previous file-event amd -handle 
            $handle->DESTROY;
            undef *README;
        }
        open(README, "./test3.pl |") or die "Can't run program: $!\n";
        $handle = $text_box->fileevent(
            \*README,
            'readable',
            sub {
                my $line = <README>;
                if($line =~ /aaa/) {
                    close README;
                    wait; # reap child
                }
                $text_box -> insert ("end", $line);
            }
        );
    }
}
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Ad 2: for calling the event loop yourself, see Re: Do I need threading for my GUI?

Note that for both solutions, you have to turn autoflush on in your external script, since output will be block buffered if you don't do so:

#!/usr/bin/perl -w
$| = 1; # make line buffered
print "Write something: \n";
while (<>) {
  print;
  if ($_ =~ /aaa/){
  exit;
  }
}
[download]

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