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

my $test = 123;

my $mw = tkinit();
my $l1 = $mw->Label(-text => $test)->pack();
$mw->Button(-command => sub {chg_txt(\$test)} )->pack;

MainLoop;

sub chg_txt {
  my $test = shift;
  $$test = 'Testtext2';
  $mw->update();
}