Note of baking calculator project - number 3

I'm continuously learning Perl/Tk by reading the book O'Reilly Mastering Perl/Tk, I finished the second chapter today which talks about the Geometry Management and 'pack' method. The key notes are as follow:

• With the exception of place, differences between the geometry managers make it difficult (not entirely impossible, but definitely not recommended) to use more than one geometry manager within the same area. ... because the two geometry managers will get into a race condition ...
• Frame within a window that uses a different geometry manager.
• With the pack geometry manager, our windows are similar to the wooden puzzle, because widgets cannot overlap or cover each other, partially or completely.
• @list = $parentwidget->packSlaves( ); packSlaves returns an ordered list of all the widgets that were packed into $parentwidget.