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();
}