Wrong question. Correct question: How do you notify Tk's thread that a property should be changed?

You could poll a Thread::Queue.

use strict;
use warnings;
use threads;

use Thread::Queue qw( );
use Time::HiRes   qw( sleep );
use Tk            qw( MainLoop );

my $q = Thread::Queue->new();

my $mw = MainWindow->new();
my $text = $mw->Entry(-width => 8)->pack();

$mw->repeat(100, sub {
   while (defined(my $command = $q->dequeue_nb())) {
      $text->delete('0.0', "end");
      $text->insert('0.0', $command);
      $mw->update();
   }
});

# Some worker.
async {
   for (;;) {
      sleep(0.2 + 1.5 * rand());
      $q->enqueue(chr(ord('A') + rand(26)));
   }
}->detach();

MainLoop();
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