You are looking for a module that is part of a bundle, here Tk::Button is a part of the Tk-Bundle.

You can easily tell that by searching the CPAN like so. When you find the module you will see the link to the modules bundle below the modules name.

Tk::Button
Create and manipulate Button widgets
Tk-804.027 - 11 Apr 2004 - Nick Ing-Simmons