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

my $main = MainWindow->new; 

my $entry = $main->Entry( -text => 'bla' ) -> pack();
$main->Button( -text => 'leer', -command => sub { $entry->configure( -text => '' ); } ) -> pack();
$main->Button( -text => 'nicht leer', -command => sub { $entry->configure( -text => 'nicht leer' ); } ) -> pack();
MainLoop; 

#!/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;