I think you've seen all possible tricks for Tk, but I will also mention that Gtk2 has a text widget that might satisfy your requirements.

#!/usr/bin/perl -w
use strict;
use Gtk2 -init;

my $window = Gtk2::Window->new;
$window->signal_connect (destroy => sub {Gtk2->main_quit});
$window->set_default_size (400, 300);

my $textview = Gtk2::TextView->new;
$window->add ($textview);

my $n;
$textview->get_buffer->signal_connect (
         changed => sub { print "changed! ".(++$n)."\n"});

$window->show_all;
Gtk2->main;
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