Hi again, I Tk is a very simple and easy to use toolkit, but
Gtk2 is more powerful but harder to use. Just for your information, here is how it's done in Gtk2.
#!/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;