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;
}