package MyPackage::Global;
use vars qw($ThisGlobalValue);
$MyPackage::Global::ThisGlobalValue = "irgendwas";

.....
package MyPackage::Callbacks;
sub ChangeThis {
 my ($widget, $index) = @_;
 $MyPackage::Global::ThisGlobalValue = $widget->value($index); 
}

my $var = "1";
$parent->Button(-textvariable => \$var, -command => [ \&UpdateButton, $button, \$var]);

my $var = "1";
my $button = $parent->Button(-text => $var);
$button->configure(-command => sub {
    $var = &UpdateValue($var);
    $button->configure('-text' => $var)
});

MyApp::Functions
MyApp::GUI
MyApp::Document

#! /usr/bin/perl
use warnings;
use strict;
use Tk;
use FindBin;
use lib $FindBin;
use MyApp::Widgets;

my $mw = MainWindow->new();
my $button = MyApp::Widgets::Button($mw, 'klick', sub { $mw->destroy() });
MainLoop();

package MyApp::Widgets;
use warnings;
use strict;

use constant BACKGROUND_COLOR => '#bfdcc0'; # als konstante
our $StandardFont = 'Arial 12 normal'; # als "globale" variable
use vars qw($BoldFont); # ebenfalls "global"
$BoldFont = 'Arial 12 normal';
my $fixedFont = "Courier 12 normal"; # oder normale my-variable

sub Button {
  my ($parent, $text, $callback, %options) = @_;
  my %options2 = ();
  if (defined $callback and ref $callback) {
    $options2{-command} = $callback;
  } # if
  else {
    $options2{-state} = 'disabled';
  }
  if (ref $text) {
    $options2{-textvariable} = $text;
  }
  else {
    $options2{-text} = $text;
  }

  return $parent->Button(
    -background => BACKGROUND_COLOR,
    -font           => $StandardFont,
    -paddx => 5, -pady => 4,
    %options2, %options,
  );
} # Button
# -----------------------------------------------------
sub Frame { ... }
#...