I think I found out the way using "geometry" method (I'm new to Tk, so did not find this method in first place)

replacing comment line with this:

    my @wg = ($w->geometry =~ /\+(\d+)\+(\d+)/);
    my @bg = ($button->geometry =~ /\+(\d+)\+(\d+)/);
    $wg[$_] += $bg[$_] for (0..@wg-1);
    $mw->geometry("+$wg[0]+$wg[1]");
[download]

has solved my problem