@a=1.10; @b=1..100; @c=1..1000; @d=1..10000;
 use Benchmark 'cmpthese';
 $ari = '@A = keys %{{ map +($_ => undef), @a, @b, @c, @d }};';

 $buk = '@B = do{local %h; ++@h{@a,@b,@c,@d}; keys %h};';

 cmpthese( -1, { ari=>$ari, buk=>$buk });
            (warning: too few iterations for a reliable count)
            (warning: too few iterations for a reliable count)
      Rate  ari  buk
ari 1.69/s   -- -28%
buk 2.36/s  39%   --

cmpthese( -3, { ari=>$ari, buk=>$buk });
      Rate  ari  buk
ari 1.81/s   -- -22%
buk 2.32/s  28%   --

cmpthese( -3, { ari=>$ari, buk=>$buk });
      Rate  ari  buk
ari 1.82/s   -- -24%
buk 2.40/s  32%   --

print 'Same' if "@A" eq "@B"
Same