has 'tree'       => ( is => 'ro',
                      isa => sub { {} },
                      default => sub { { '/' => { children => {} } } } );

has 'clean_tree' => ( is => 'rw',
                      lazy => 1,
                      predicate => 1,
                    );

sub BUILD {
  my $s = shift;

  $s->app->add_hook (Dancer2::Core::Hook->new (
    name => 'before_template',
    code => sub {
      ...
      if (!$s->has_clean_tree) {
        my %tree = %{$s->tree};   # Attempting to make shallow copy here.
        $s->clean_tree(\%tree);
      }

      # set active
      my $tree = $s->clean_tree->{'/'};
      foreach my $segment (@segments) {
        $tree->{children}{$segment}{active} = 1;
        $tree = $tree->{children}{$segment};
      }

      ...
    }
  ));
}