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return keys %{ {map { $_ => 1 } @_} };

say Dumper(\%{ {map { $_ => 'fish' } qw(one two red blue)} } );
             ^ ^                                         ^ ^
             |-|-----------------------------------------|-|

$VAR1 = {
          'blue' => 'fish',
...
          'red' => 'fish',
          'two' => 'fish'
        };

Ambiguous use of %{map{...}} resolved to %map{...}

$VAR1 = [
          'one',
...
          'blue',
          'fish'
        ];

say Dumper(\@{ {map { $_ => 'fish' } qw(one two red blue)} } );

Not an ARRAY reference