{ A => "b" }
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creates a hash and returns a reference to it, like

do { my %anon = ( A => "b" ); \%anon }
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So you are doing

my %anon = ( A => "b" );
my %keys = \%anon;
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When you assign to a hash, it expects a list of key-value pairs. You just provide one item, so it's use as a key and undef is used as a value. (This would have given you a warning if you had them on!)

So you are doing

warn("Odd number of elements in hash assignment");
my %anon = ( A => "b" );
my %keys = ( \%anon => undef );
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To return the contents of a referenced hash, you need to dereference it.

my %keys = %{  { A => "b" }  };
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But there's no reason to construct the anonymous hash at al. You could just use

my %keys = ( A => "b" );
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