use warnings;
use strict;

use Tk;

my $mw = MainWindow->new(); # oder new(-background => '#FFFFFF')
my $b_yel = $mw->Button(
    -command => [sub{$mw->configure(-background => '#FFFF00')}],
    -text    => 'Gelb'
)->pack();
my $b_mag = $mw->Button(
    -command => [sub{$mw->configure(-background => '#FF00FF')}],
    -text    => 'Magenta'
)->pack();

$mw->configure(-background => '#FFFFFF');

MainLoop();