Endless loop in Mojo::DOM caused by root going out of scope

Mojolicious version: 9.22
Perl version: 5.34.0
Operating system: Ubuntu 18.04.6 LTS

Steps to reproduce the behavior

The following works as expected:

use Mojo::DOM;
my $dom = Mojo::DOM->new('<div><a></a><b></b></div>')->at('div')
    ->children('a')->map( sub { say $_->tag } );
# OUTPUT:
# a

However, when storing the DOM in an intermediate variable, the code instead goes into an endless loop when doing a string comparison against an undefined value:

use Mojo::DOM;
my $dom = Mojo::DOM->new('<div><a></a><b></b></div>')->at('div');
$dom->children('a')->map( sub { say $_->tag } );
# OUTPUT:
# Use of uninitialized value in string ne at .../Mojo/DOM58/CSS.pm line 259.
# ... endless loop ...

Note that I have only seen this affect calls to ->children with a selector. If no selector is given, then this error is not triggered:

use Mojo::DOM;
my $dom = Mojo::DOM->new('<div><a></a><b></b></div>')->at('div');
$dom->children->map( sub { say $_->tag } );
# OUTPUT:
# a
# b

Possible diagnosis

The warning points to the _root sub in Mojo::DOM::CSS

sub _root {
  my $tree = shift;
  $tree = $tree->[3] while $tree->[0] ne 'root'; # <-- BOOM
  return $tree;
}

The ne the warning complains about is the one trying to find the root of the tree, which has gone out of scope. This suggested the following workaround, which does allow the code to behave as expected:

use Mojo::DOM;
my $dom = Mojo::DOM->new('<div><a></a><b></b></div>');
my $root = $dom->root; # Keep it in scope, must be above next line
$dom = $dom->at('div');
$dom->children('a')->map( sub { say $_->tag } );
# OUTPUT:
# a

Expected behavior

I expected that storing a reference to the DOM after calling ->at would give me an instance that I could use in the same way as the full DOM, except at a different point in the tree.

Actual behavior

Some calls on the intermediate object triggered an endless loop.

mojolicious / mojo