use Benchmark;

my @keys = qw(a b c d e f g h i j);

sub _exists {
   my %keys;
   exists $keys{$_}{1}
      for @keys;
}

sub _keyref {
   my %keys;
   $keys{$_} = {}
      for @keys;
}

Benchmark::cmpthese(-1, {
   'exists' => \&_exists,
   'key=ref' => \&_keyref,
});