use Benchmark qw(:all) ;

my @a;
push @a, int (rand(100)) foreach 1..2_000_000;

my %unique;
my (@awd1, @awd2, @awd3);
cmpthese(5, {
                  'jc' =>
                  sub { foreach my $thingy (@a) { $unique{$thingy} = 1; }
                        @awd1 = keys %unique;
                     },
                  'mk' =>
                  sub { @unique{ @a} = 1;
                        @awd2 = keys %unique;
                     },
                  'ys' =>
                  sub { @unique{ @a} = ();
                        @awd3 = keys %unique;
                     },
                 });

##</code><code>##

Benchmark: timing 5 iterations of jc, mk, ys...
        jc: 19 wallclock secs (16.75 usr +  0.35 sys = 17.10 CPU) @  0.29/s (n=5)
        mk:  6 wallclock secs ( 6.00 usr +  0.01 sys =  6.01 CPU) @  0.83/s (n=5)
        ys:  7 wallclock secs ( 6.00 usr +  0.00 sys =  6.00 CPU) @  0.83/s (n=5)
   s/iter   jc   mk   ys
jc   3.42   -- -65% -65%
mk   1.20 185%   --  -0%
ys   1.20 185%   0%   --