The reason why is because your map is evaluating into a hash reference, not a hash - values take a hash for it's argument, not a hash reference. Now, if you use a tempory hash reference instead, the reason for the double curlies becomes much more apparent:

my $hash   = { map {$_ => $_ } @a,@b };
my @merged = values %{ $hash };
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Good question, by the way :)

jeffa