By calling MainLoop, the program doesn't continue, it waits for the main window to stop. You can add events to the loop, though: for example, if you want to run something immediately after the creation of the window, you can use after(0,...):

$mw->after(0, sub { doSomething($mw) });
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Be sure to place this before calling the main loop! It tells the main window to call the given subroutine after 0 miliseconds once the main loop is entered.