use strict;
use warnings;
use Tk;
my $mw = Tk::MainWindow->new();
$mw->Button( -text => "Exit", -command => sub { $mw->destroy(); }
             )->pack();
# or
$mw->Button( -text => "Exit", -command => \&pressExitButton
             )->pack();
MainLoop();
sub pressExitButton {
  $mw->destroy();
}

##</code><code>##

package MyGui;
use strict;
use warnings;
use Tk;

sub new
{
  my $this  = shift;
  my $class = ref($this) || $this;
  my $self  = {};
  bless $self,$class;
  $self->buildInterface();
  return $self;
}

sub buildInterface
{
  my($self) = @_;
  my $mw = $self->_mainWindow( Tk::MainWindow->new() );
  $self->_exitButton(
    $mw->Button(
      -text => "Exit",
      -command => sub { $self->pressExitButton(@_); }
    )
  )->pack();
  return 1;
}

sub pressExitButton
{
  my($self) = @_;
  $self->_mainWindow()->destroy();
}

sub run
{
  my($self) = @_;
  MainLoop();
}

sub _mainWindow {
  my($self,$arg) = @_;
  if($arg) { $self->{'_mainWindow'} = $arg; }
  return $self->{'_mainWindow'};
}

sub _exitButton {
  my($self,$arg) = @_;
  if($arg) { $self->{'_exitButton'} = $arg; }
  return $self->{'_exitButton'};
}

1;
package main;
MyGui->new()->run();