#! perl -slw
use strict;
use vars qw[@a @b @c @d];
use Benchmark 'cmpthese';

our @a=1..10; our @b=5..15; our @c=10..20; our @d=15..25;
our (@A, @B, @C);

my $ari = '@A = keys %{{ map +($_ => undef), @a, @b, @c, @d }};';
my $buk = '@B = do{local %h; ++@h{@a,@b,@c,@d}; keys %h};';
my $buk2= '@C = do{   my %h; ++@h{@a,@b,@c,@d}; keys %h};';

cmpthese( -1, { ari=>$ari, buk=>$buk, buk2=>$buk2 });

print "@A\n@B\n@C";
__END__
C:\test>junk
Benchmark:
running ari, buk, buk2, each for at least 1 CPU seconds
...
       ari:  1 wallclock secs ( 1.07 usr +  0.00 sys =  1.07 CPU) @ 1252.80/s (n=1343)
       buk:  1 wallclock secs ( 1.16 usr +  0.00 sys =  1.16 CPU) @ 2312.39/s (n=2687)
      buk2:  1 wallclock secs ( 1.09 usr +  0.00 sys =  1.09 CPU) @ 2462.88/s (n=2687)

       Rate  ari  buk buk2
ari  1253/s   -- -46% -49%
buk  2312/s  85%   --  -6%
buk2 2463/s  97%   7%   --
1 2 3 4 10 5 11 20 6 12 21 7 13 22 8 14 23 9 15 24 16 25 17 18 19
1 2 3 4 10 5 11 20 6 12 21 7 13 22 8 14 23 9 15 24 16 25 17 18 19
1 2 3 10 4 11 5 20 12 6 21 13 7 22 14 8 23 15 9 24 16 25 17 18 19

C:\test>perl58 junk.pl
       Rate  ari  buk buk2
ari   744/s   -- -41% -43%
buk  1265/s  70%   --  -3%
buk2 1307/s  76%   3%   --
11 21 7 17 2 22 1 18 23 16 13 25 6 3 9 12 20 14 15 8 4 24 19 10 5
11 21 7 17 2 22 1 18 23 16 13 25 6 3 9 12 20 14 15 8 4 24 19 10 5
11 21 7 17 2 22 1 18 23 16 13 25 6 3 9 12 20 15 14 8 4 24 19 10 5