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;
                     },
                 });

##</code><code>##

Benchmark: running jc, mk for at least 3 CPU seconds...
        jc:  3 wallclock secs ( 3.14 usr +  0.00 sys =  3.14 CPU) @ 535.67/s (n=20522)
        mk:  4 wallclock secs ( 3.27 usr +  0.00 sys =  3.27 CPU) @ 14937.00/s (n=48844)
      Rate   jc   mk
jc  6536/s   -- -56%
mk 14937/s 129%   --