You are ecncountering a problem with threads:shared hashes where only the first level keys get shared by default. When you run as shared, the $top_hash{A}{B} gets ignored as being shared, while it runs fine in normal perl.

To amplify a bit, the Tk eventloop will never respond to a change in threads shared variables, when they are across thread boundaries. You don't have actual threads here, but you need to setup a timer, to read for any changes in the shared variable. So you can't use tied variables effectively across threads without timers. Try this:

#!/usr/bin/perl
use warnings;
use strict;
use threads;
use threads::shared;
use Tk;

my %top_hash;

share $top_hash{'A'}{'B'};
$top_hash{'A'}{'B'} = 'Original';

my $t = MainWindow->new()->Scrolled('Text')->pack();
my $e = $t->Entry(-textvariable => \$top_hash{'A'}{'B'});
$t->windowCreate('end', -window => $e);
$t->insert('end', "\n");

my $timer = $t->after(3000,sub{$top_hash{'A'}{'B'} = 'Changed'});

MainLoop;
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