Well, I think the point is that map isn't primarily meant to operate on hashes, it takes a list and returns a list.

LIST = map {BLOCK} LIST

But assigning a list to a hash can result in collisions

  DB<166> %hash = (Canberra => { 12 => 1}, Canberra => { 18 => 1} )
 => ("Canberra", { 18 => 1 })
[download]

You could however design a sub join_deep , such that:

  DB<178> sub join_deep {
             my %hash;
             while ( my ($a,$b) = splice @_,0,2 ) {
                $hash{$a}= { %{$hash{$a}}, %$b }
             }
             return %hash;
          }

  DB<179> %h =join_deep (Canberra => { 12 => 1}, Canberra => { 18 => 1
+} )
 => ("Canberra", { 12 => 1, 18 => 1 })
[download]

But frankly, I don't see the need for such abstraction.