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

use Tk;

my $main = tkinit;
my $label = $main->Label(-text => 'Hallo')->pack();
$main->Button(-text => 'change', -command => [\&foo,$label])->pack();

MainLoop();

sub foo {
  my ($label) = @_;
   $label->configure(-text => 'blubb')->pack();
}