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

In reply to Re^2: Perl/Tk widget coordinates, simple question by warpod
in thread Perl/Tk widget coordinates, simple question by warpod

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