Lets look at it blown out:

= values %{{ map {$_ => $_ } @a,@b }} ;
= values                       # I want a hash and return a list
         %{                  } # I turn a hashref into a hash
            {              }   # I turn a list into a hashref
             map {  } @a,@b    # I return a list from a list
                  =>    ,      # We construct lists
[download]

HTH...

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