Hi, the same thing happens on Linux depending on the window manager you use. Where a window pops up depends on the settings in the window manager. If you don't specify exact coordinates, the window manager "guesses" as to the best place to put it. Some people like "always upper right corner", some like "where the mouse pointer is", etc.

You can also use negative numbers to specify start from the lower right corner.

$mw->geometry('100x100-0-0');
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Here is a useful sub to center a window on any screen, as it automatically detects monitor size.

sub CenterWindow
{
    my($window, $width, $height) = @_;
    $window->idletasks;
    $width  = $window->reqwidth  unless $width;
    $height = $window->reqheight unless $height;
    my $x = int(($window->screenwidth  / 2) - ($width  / 2));
    my $y = int(($window->screenheight / 2) - ($height / 2));
    $window->geometry($width . "x" . $height . "+" . $x . "+" .$y);
}
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