#!/usr/bin/perl
package dragwin;

use strict;
use diagnostics;
use warnings;

use Gtk2 '-init';

# Want to detect when the user drags a window across the desktop
# Specifically, when the user releases their mouse button, meaning the
#    drag operation has finished

my ($mouse_x, $mouse_y, $mask);
my ($window_x, $window_y);

# Create the window
my $window = Gtk2::Window->new();

$window->set_title('Drag me!');
$window->set_position('center');
$window->set_default_size(300, 300);
$window->set_border_width(5);
   
$window->signal_connect (destroy => sub { Gtk2->main_quit; });

# We want to detect the end of a drag, but this doesn't work
$window->signal_connect('event' => sub {

  my ($widget, $event) = @_;

  print "Window event type: " . $event->type . "\n";    
  exit 0 if $event->type eq "delete";

  # Get mouse position    
  (undef, $mouse_x, $mouse_y, $mask)
      = Gtk2::Gdk::Screen->get_default->get_root_window->get_pointer;

  # Get Windows (top-left) position 
  ($window_x, $window_y) = $window->get_position;

  # print current positions
  print "POS($mouse_x,$mouse_y) - ($window_x, $window_y)\n";

});

# Open the window
$window->show_all();
Gtk2->main();
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See a perlmonk post here: node_id=980493 and the GTK Window webpage