Tk apparently has two separate hierarchies, parent/child and master/slave. And in my vague and limited understanding, the first controls stacking order and the second controls geometry (screen space). So taking a wild guess here, trying to repack a button with first tab parent into a second tab, changes the geometry but not the parent/child relationship -- meaning the stacking wouldn't work. Take that with a grain of salt, but if you change your button to be a child of the main window, your code works:

my $t_button = $mw->Button(-text=>'OK',
                           -command=>sub{exit});
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