Your problem is mixing the geometry managers Tk::grid and Tk::pack in the one frame (in this case, the special frame Tk::MainWindow).

If you want to mix them, put the first five widgets in their own Tk::Frame.

I changed the lone pack() to grid() and your reported problem was resolved, i.e. a window now appears.

my $output = $mw -> Text(-width=>500, -height=> 240) -> grid();
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I didn't check any functionality beyond this.