use strict;
use Tk;

my $mw = MainWindow->new;
$mw->overrideredirect(1);

my $frame = $mw->Frame->
    pack(-padx => 20, -pady => 20);

$frame->Button(
    -text => 'Exit',
    -command => sub { Tk::exit }
)->pack;

my ($lastX, $lastY);

$mw->bind('<Button1-Motion>',
    [\&MoveWindow, Ev('X'), Ev('Y'), \$lastX, \$lastY]);

$mw->bind('<ButtonRelease-1>',
    [\&ClearCoords, \$lastX, \$lastY]);

MainLoop;

sub ClearCoords
{
    my ($cw, $xSR, $ySR) = @_;
    undef($$xSR);
    undef($$ySR);
}

sub MoveWindow
{
    my ($mw, $rootX, $rootY, $xSR, $ySR) = @_;

    if (!defined($$xSR) && !defined($$ySR)) {
       $$xSR = $rootX;
       $$ySR = $rootY;
    }
    else {
       my $xDiff = $rootX - $$xSR;
       my $yDiff = $rootY - $$ySR;

       my $x = $mw->x + $xDiff;
       my $y = $mw->y + $yDiff;
       $x = "+".$x if $x >= 0;
       $y = "+".$y if $y >= 0;

       $mw->geometry(sprintf("%dx%d%s%s",
          $mw->width,
          $mw->height,
          $x,
          $y
       ));
       $$xSR = $rootX;
       $$ySR = $rootY;
    } 
}