use strict;
use warnings;
use Benchmark qw(cmpthese);

my @XYZ = qw(Mary Mary Mary Mary Joe Joe Joe);

cmpthese
  (
  -1,
    {
    'GF' => sub {my %unique; @unique{@XYZ} = (); keys %unique;},
    'DW' => sub {keys %{{map {$_=>undef} @XYZ}};},
    'RJ' => sub {keys %{(grep \@{$_}{@XYZ}, {})[0]};},
    }
  );

##</code><code>##

       Rate   DW   RJ   GF
DW  71543/s   -- -62% -73%
RJ 188025/s 163%   -- -28%
GF 260808/s 265%  39%   --