use strict;
use warnings;
use Data::Dumper;

my %hash;

print "key foo->bar exists\n"
   if exists $hash{foo}{bar};

print Dumper(\%hash);

my %hash;

print "key foo->bar exists\n"
  if(exists($hash{foo}) and exists($hash{foo}{bar}));

use strict;
use warnings;
use Data::Dumper;

my %hash;
my $tmp;

print "key foo->bar exists\n"
  if $tmp = $hash{foo} and exists $tmp->{bar};

print Dumper(\%hash);

use strict;
use warnings;
use Data::Dumper;

my %hash;
my $tmp;

print "key foo->bar exists\n"
  if $tmp = $hash{foo} and exists $tmp->{bar};

print Dumper(\%hash);

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,
});