my $doStdout = sub {
         ## $Qout is a [Thread::Queue] object onto which anything that is printed to stdout is enqued by the "black magic".
         ## It is created by the statement <c>my $Qout = new Thread::Queue;</c>.
         ## If you look up the documentation for that module you'll see that the <c>pending()</c> method returns 
         ## the count of 'things' in the the queue. Using that information, the next line of code reads:

         ## If there is anything in the queue -- ie. if the parent program has printed anything to stdout
         if( $Qout->pending ) {    # no clue

             my $output = $Qout->dequeue;    # read STDOUT of .pl into var   ## YES.
             $lb->insert( 'end', $output ) ;    # print STDOUT of .pl in listbox  ## YES.

             ## We've inserted the next item into the listbox,
             ## but this addition may have gone 'off the bottom' of the visible part of the listbox
             ## so ask the listbox to redraw itself to ensure that we can 'see' the 'end'
             ## ie. the last item in the listbox. Ie. the one we just added.
             $lb->see( 'end' );                # What's this?

                 if( $output eq 'whatever' ) {
                 # actual Tk code 
             }
         }
     };

##</code><code>##

    $mw->repeat( 500, $doStdout ); # execute $doStdout once every 500 millisec...? but why mw->

     Tk::MainLoop(); #OK
}

##</code><code>##

use Thread::Queue;

 ## A queue, that we tie to the main programs stdin.
 ## Anything our Tk code <c>$Qin->enqueues( ... )</c> (writes) to this queue
 ## Will turn up in the main program when it reads from stdin (Eg. <c>while( my $line = <STDIN> )</c> etc.)
 my $Qin  = new Thread::Queue;

 ## And another queue that we tie to the main programs stdout.
 ## Anything the main program prints to stdout, we can access in our Tk code 
 ## by reading (<c>my $line = $Qout->dequeue()</c>) from this queue
 my $Qout = new Thread::Queue;

 ## So, we've created the means for bidirectional communications between us and the main program
 ## And now we need to (transparently) persuade the main program to use them instead of the console.
 ## Which we do by [tie]ing the standard handles to our newly created queues.

 tie *STDIN,  'MyGuiStdin',  $Qin;
 tie *STDOUT, 'MyGuiStdout', $Qout;

 # everything up to this point is pure black magic

##</code><code>##

sub PRINT  { $_[0]->enqueue( join ' ', @_[ 1 .. $#_ ] ); }

##</code><code>##

package MyGuiStdin;
our @ISA = qw[ Thread::Queue ];

sub TIEHANDLE { bless $_[1], $_[0]; }
sub READLINE { $_[0]->dequeue(); }