This is easier to pull off than that I thought. HTML::TreeBuilder already makes allowance for building a tree with objects other than instance of HTML::Element.

my $tree = new HTML::TreeBuilder(...);
# now replace 'HTML::Element' with my subclass
$tree->{'_element_class'} = 'HTML::MyElementSubclass';
[download]

Or, if this offends your sensibilities, subclass HTML::TreeBuilder, override new(), and invoke INHERITED::new() before resetting the element class.

This works with HTML::TreeBuilder 3.09.