There may be a way to change the existing module from your program, by overriding the existing module's sub, but it would probably generate a "redefined" error. The more proper "Perl way" would be to make your own version of the MenuButton module, and call it say "MyMenuButton.pm".

You can do it 2 ways. One is to manually edit a copy of the module, and rename it to MyMenuButton.pm and place it somewhere in your PERL5LIB path.

Two, and the more elegant way, is to make a derived widget from the MenuButton module. Google for "perl tk derived widget" for numerous tutorials and examples for derived widgets. See Making a derived Tk::Text object for a simple example.