use Benchmark qw(:all) ;

my @a;
push @a, int (rand(10)) foreach 1..100;
my %unique;
my @awd;
cmpthese($1000, {
                  'jc' =>
                  sub { foreach my $thingy (@a) { $unique{$thingy} = 1; }
                        @awd = keys %unique;
                     },
                  'mk' =>
                  sub { @unique{ @a} = 1;
                        @awd = keys %unique;
                     },
                   'bt' =>
                  sub { undef(@unique{@a});
                        @awd = keys %unique;
                     },
                 });

##</code><code>##

      Rate   jc   mk   bt
jc 16556/s   -- -54% -62%
mk 36080/s 118%   -- -18%
bt 44007/s 166%  22%   --