use Benchmark 'cmpthese';
use strict;

my @x = map join(',', split(//, rand(10000))), 0..80000;
my %y;

cmpthese(-2,
         {'@y{@x}'   => sub { undef %y; @y{@x} = undef },
          '%y = map' => sub { undef %y; %y = map(($_ => undef), @x) }
         }
        );

print 0+%y,$/;