gtk_hello_world.pl (improved)

#!/usr/bin/perl -w

use strict;
use Gtk2 -init;

my $window = Gtk2::Window->new;
$window->signal_connect(
  delete_event => sub { Gtk2->main_quit; }
);

my $label = Gtk2::Label->new('Hello World!');
$window->add($label);

$window->show_all;

Gtk2->main;