$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);
            }
        );
    }
}