The HTML::TreeBuilder documentation is a good place to start. It says that elementify ():

This changes the class of the object in $root from HTML::TreeBuilder to the class used for all the rest of the elements in that tree (generally HTML::Element). Returns $root.

and goes on to say:

For most purposes, this is unnecessary, but if you call this after (after!!) you've finished building a tree, then it keeps you from accidentally trying to call anything but HTML::Element methods on it. ...