Try the documentation for Tk::mega and Tk::composite. The first is probably more helpful.

Tk supports both derived widget types ("make me a widget that's just like Text, only can also do...", e.g. <samp>Tk::LabEntry</samp>) and composite widget types ("make me a new type of widget, that is implemented in terms of sub-widgets", e.g. <samp>Tk::FileSelect</samp>). The Tk::mega documentation covers both, which might be a bit confusing.