diamondsandperls:
Remember that  Tk (and, as far as I understand, any GUI) is an event-driven system. Once program execution enters the  MainLoop() function, the only things that count are events:  Button presses, keypresses on the keyboard, mouse movements and button clicks, etc., etc. The keyboard keypresses that would normally be available via  STDIN are  MainLoop events. Keyboard keypresses can go to a  Text widget, an  Entry widget, etc., if the widget is in focus. Otherwise...
(Update: However, I think (without testingsee subsequent post) it should be possible to capture  STDIN data before the  MainLoop is entered provided the  Tk script is launched from the command line. If it is launched via an icon click, we're back in territory unfamiliar to me.)

I know how to re-direct  STDOUT and  STDERR so as to make functions like  print() and  printf() output to, e.g., a  Text box. I do not know how to do anything similar with  STDIN and I am not even sure it is meaningful to talk of doing such a thing (Update: from within the  MainLoop)!

Can you describe in greater detail what it is you hope to accomplish with your program? What is the nature of the data that  STDIN is supposed to provide?

After playing around a bit, here's an example of  STDIN accessibility both before and after the  MainLoop function. I don't know how kosher this may be. Again, this code is launched from the command line.

>perl -wMstrict -le
"use Tk;
 ;;
 my $text;
 ;;
 my $mw = MainWindow->new;
 ;;
 $mw->Label(-text => 'Enter text: ')->pack();
 $mw->Entry(-textvariable => \$text)->pack();
 $mw->Button(-text => 'Submit', -command => sub { $mw->destroy }
   )->pack();
 ;;
 print qq{before MainLoop: enter text: };
 my $pre_ml = <STDIN>;
 chomp $pre_ml;
 ;;
 MainLoop;
 ;;
 seek STDIN, 0, 2;
 print qq{after MainLoop: enter more text: };
 my $post_ml = <STDIN>;
 chomp $post_ml;
 ;;
 print qq{before MainLoop: '$pre_ml'};
 print qq{in MainLoop: '$text'};
 print qq{after MainLoop: '$post_ml'};
"
before MainLoop: enter text:
Before
after MainLoop: enter more text:
After all
before MainLoop: 'Before'
in MainLoop: 'hi there'
after MainLoop: 'After all'
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You must destroy the MainLoop with $mw->destroy for processing to continue.

$mw->Button(
        -text => "Submit",
        #-command => sub {exit} # no good, will exit whole program
         -command => sub{ $mw->destroy }   # good 
)->pack();
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You can then take $text, and run any code you want using it.

However, if you have an external program, which you want to feed $text to on it's STDIN, you have a few choices.

One is to run system or exec, with $text as an argument. Like this:

....  Tk code omitted
MainLoop;
# after running $mw->destroy, the following lines will get processed h
+ere, right after the MainLoop line

# if you have an external program to feed $text to, try one of these

system( "path_to_program  $text");
#or
exec ( "path_to_program  $text");

# if you want to run your program and get ouput

use IPC::Open3;
my $pid = open3(\*WRITE, \*READ, \*ERROR,"your_command");
if( ! $pid ){ die "$!\n";}

#send $text to your_command's STDIN
print WRITE "$text\n";

#get the answer from your_command
chomp(my $answer = <READ>);
print "response = $answer\n";

#get the error from your_command
chomp(my $error = <ERROR>);
print "error = $error\n";
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