Care must be used with exists as it will indeed autovivify intermediate hashes: 

use strict; use warnings; use feature 'say';
use Data::Dumper; $Data::Dumper::Sortkeys = $Data::Dumper::Indent = 1;

my $href = {
    cat => { milk => 1 },
    dog => { bone => 1 },
};

say exists $href->{'cow'}->{'alfalfa'} ? 'cow' : 'no cow';

say Dumper $href

__END__

[download]
Output: 
no cow
$VAR1 = {
  'cat' => {
    'milk' => 1
  },
  'cow' => {},    # uh-oh
  'dog' => {
    'bone' => 1
  }
};

[download]
So you would have to either use exists on all levels of the structure as haukex suggested: 
use strict; use warnings; use feature 'say';
use Data::Dumper; $Data::Dumper::Sortkeys = $Data::Dumper::Indent = 1;

my $href = {
    cat => { milk => 1 },
    dog => { bone => 1 },
};

say exists $href->{'cow'} &&
    exists $href->{'cow'}->{'alfalfa'} ? 'cow' : 'no cow';

say Dumper $href

__END__

[download]
Output: 
no cow
$VAR1 = {
  'cat' => {
    'milk' => 1
  },
  'dog' => {
    'bone' => 1
  }
};

[download]
... or use autovivification: 
use strict; use warnings; use feature 'say';
use Data::Dumper; $Data::Dumper::Sortkeys = $Data::Dumper::Indent = 1;

my $href = {
    cat => { milk => 1 },
    dog => { bone => 1 },
};

no autovivification;
say exists $href->{'cow'}->{'alfalfa'} ? 'cow' : 'no cow';
say Dumper $href

__END__

[download]
Output: 
no cow
$VAR1 = {
  'cat' => {
    'milk' => 1
  },
  'dog' => {
    'bone' => 1
  }
};

[download]
Note that autovivification.pm has effect lexically: 
use strict; use warnings; use feature 'say';
use Data::Dumper; $Data::Dumper::Sortkeys = $Data::Dumper::Indent = 1;

my $href = {
    cat => { milk => 1 },
    dog => { bone => 1 },
};

{
    no autovivification;
    say exists $href->{'cow'}->{'alfalfa'} ? 'cow' : 'no cow';
    say Dumper $href
}

say exists $href->{'cow'}->{'alfalfa'} ? 'cow' : 'still no cow';
say Dumper $href;

__END__

[download]
no cow
$VAR1 = {
  'cat' => {
    'milk' => 1
  },
  'dog' => {
    'bone' => 1
  }
};

still no cow
$VAR1 = {
  'cat' => {
    'milk' => 1
  },
  'cow' => {},    # uh-oh
  'dog' => {
    'bone' => 1
  }
};

[download]


The way forward always starts with a minimal test.

In reply to Re^2: keys %{$hash->{$href}} adds $href to the hash if it doesnt exist? by 1nickt
in thread keys %{$hash->{$href}} adds $href to the hash if it doesnt exist? by nikmit

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