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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

$VAR1 = [
          'one',
          'fish',
          'two',
          'fish',
          'red',
          'fish',
          'blue',
          'fish'
        ];

##</code><code>##

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

##</code><code>##

Not an ARRAY reference