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    my $doStdout = sub {
         ## $Qout is a [Thread::Queue] object onto which anything that
+ is printed to stdout is enqued by the "black magic".
...
             }
         }
     };

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

     Tk::MainLoop(); #OK
}

use Thread::Queue;

...
 tie *STDOUT, 'MyGuiStdout', $Qout;

 # everything up to this point is pure black magic

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

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

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