I'm wondering if this relates to perl having difficulty distinguishing between the qw## operator versus a bare hash key that looks like qw##.

B::Deparse to the rescue:

$ perl -MO=Deparse -wE '$foo{a}=1; $foo{b}=2; say join",", @foo{qw#a b
+#}'
BEGIN { $^W = 1; }
BEGIN {
    $^H{'feature_unicode'} = q(1);
    $^H{'feature_say'} = q(1);
    $^H{'feature_state'} = q(1);
    $^H{'feature_switch'} = q(1);
}
$foo{'a'} = 1;
$foo{'b'} = 2;
say join(',', @foo{'a', 'b'});
-e syntax OK
[download]

So it deparses correctly to @foo{'a', 'b'}

When I used / instead of # as a delimiter, I get the same deparse, and no warning on running. So I don't see parsing issues as the reason for this bug.