Weird Iteration/Switch Bug

desc.txt

Okay, so I am at a loss so far as to what is happening here, but something
broke. 

In Template::Anti, the node-set.t test is failing:

    t/node-set.t ........... 
    ok 1 - text works
    ok 2 - attrib one works
    ok 3 - attrib two works
    ok 4 - attrib three works
    not ok 5 - 
    # Failed test at t/node-set.t line 43
    # expected: '<root><one/></root>'
    #      got: '<root><one/><two/><three/></root>'
    not ok 6 - 
    # Failed test at t/node-set.t line 53
    # expected: '<root><one href="http://example.com/vader">Vader</one><one href="http://example.com/sidious">Sidious</one></root>'
    #      got: '<root><one href="http://example.com/vader">Vader</one><two/><three/><one href="http://example.com/sidious">Sidious</one></root>'
    ok 7 - modifies original
    1..7
    # Looks like you failed 2 tests of 7
    Dubious, test returned 2 (wstat 512, 0x200)
    Failed 2/7 subtests 

(See https://travis-ci.org/zostay/Template-Anti#L1104 for the original.)

This is failing because the truncate() method in NodeSet.pm6 is failing to
remove <two/> and <three/> from the DOM as is expected. The code for that looks
like this:

    method truncate(Int $keep = 0) {
        for @!nodes -> $node {
            my $kept = 0;
            if $keep == 0 {
                $node.nodes = ();
            }
            else {
                for $node.nodes {
                    when XML::Element { .remove if $kept++ >= $keep }
                    when XML::Node    { .remove }
                    default           { }
                }
            }
        }
        self
    }

(See
https://github.com/zostay/Template-Anti/blob/master/lib/Template/Anti/NodeSet.pm6#L177
for the original.)

The problem is that when it runs the innter loop (for $node.nodes), the block
for the `when XML::Element condition` is not being run even though there are
XML::Element nodes present. They are not falling into the XML::Node branch
either (obviously, since they would be removed if there were).

However, changing the for-loop like this instead fixes the problem:

    my @nodes = $node.nodes;
    for @nodes {

The question is why? Is there something wrong with using $node.nodes like this
or is something wrong with Rakudo? Or have I overlooked something else I have
done wrong here?

For some reason I was expecting (and pre-GLR, this assumption seems to have held true) that the for-loop iterated over a copy of the array. I was also under the impression that Perl 5's for-loop did the same. However, this is not true. When items in the array are deleted using .remove, some elements where being skipped during iteration.

Compare:

my @foo = 1,2,3; for @foo { .say; @foo.splice(0, 1) }

to:

my @foo = 1,2,3; for @=@foo { .say; @foo.splice(0, 1) }

The latter was basically what I was expecting to happen.