#!/usr/bin/perl
use Tk;
use strict;
use warnings;

my $main = MainWindow->new;
my $var  = 'bla';
my $entry = $main->Entry( -textvariable => \$var ) -> pack();
$main->Button( 
    -text => 'leer', 
    -command => sub { $var = q~~ } 
) -> pack();

$main->Button(
    -text => 'nicht leer', 
    -command => sub { $var = 'nicht leer' } 
) -> pack();
MainLoop;