Aha! perl5200delta:

The warning "Scalar value @hash{foo} better written as $hash{foo}" now produces far fewer false positives. In particular, @hash{+function_returning_a_list} and @hash{ qw "foo bar baz" } no longer warn. The same applies to array slices. [perl #28380, #114024]

The commit is 429a25554a6. The argument for this change was that something like this is useful:

use warnings;
use strict;
my @foo;
$foo[0] = "abcd"=~/b(.)/; # scalar context
@foo[1] = "abcd"=~/b(.)/; # list context
print "{$_}\n" for @foo;
__END__
{1}
{c}
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