#!/usr/bin/perl -w 
use Tk; 
use strict;

my $text = "foo";

my $mw = MainWindow->new; 

my $button = $mw->Button(-text=>'Change entry', -command=>\&change)->pack();
my $entry = $mw->Entry(-textvariable=>$text)->pack();

MainLoop;

sub change
{
	print "\$text before: $text\n";
	$text = "bar";
	print "\$text after: $text\n";
	$mw->update;
}