Here's a version without both \K and the /r option to the s///.
It should run on older perls.

It also uses B1-Motion to avoid an extra flag.

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1196818

use strict;
use warnings;
use Tk;

sub xy { $_[0]->XEvent->x, $_[0]->XEvent->y }

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

$mw->Label(
  -text => 'Press 1, Move, then Release in green Canvas to move window
+',
  )->pack(-fill => 'x');

my $c = $mw->Canvas(-width => 500, -height => 400,
  -bg => 'green',
  )->pack;

$c->Tk::bind('<1>' => \&leftdown);
$c->Tk::bind('<B1-Motion>' => \&leftmotion);

$mw->Button(-text => 'Exit', -command => sub { $mw->destroy },
  )->pack(-fill => 'x');

MainLoop;

my ($startx, $starty);

sub leftmotion
  {
  my ($endx, $endy) = &xy;
  my $deltax = $endx - $startx;
  my $deltay = $endy - $starty;
  my ($size, $x, $y) = split /\+/, $mw->geometry;
  $mw->geometry( "$size+" . ($x + $deltax) . "+" . ($y + $deltay) );
  }

sub leftdown
  {
  ($startx, $starty) = &xy;
  }
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