I got this far before deciding I should see if I'm re-inventing the wheel:

sub wrap {
  my $tree = HTML::TreeBuilder->new_from_content(shift);
  foreach my $h2 ( $tree->find_by_tag_name('h2') ) {
    my $parent = HTML::Element->new('div', 'data-role' => 'collapsible
+');
    $h2->replace_with($parent);
    $parent->push_content($h2);
  }
  return $tree->as_HTML;
}
[download]

The above only wraps the <h2> tag, not its associated content. The HTML is generated from markdown using Markdent so I think the proper thing to do is figure out how that works. Seems like it can do the job.