In Perl keys get defined if they are accessed as hash, without checking for their existence 


In the following code snippet, 'c' is not as key in 'foo' but is set to {} when the last check for foo->c->0 is called.


/tmp()$ cat foo.pl
use strict;
use warnings;
use Data::Dumper;

sub abcde {
   my $foo = {'a' => 1, 'b' =>2};
   print Dumper($foo);

   if (exists $foo->{c} && defined $foo->{c}->{0}) {
      print "C is defined";
   }
   print Dumper($foo);

   if (defined $foo->{c} and defined $foo->{c}->{0}) {
      print "C is defined";
   }
   print Dumper($foo);

   if (defined $foo->{c}->{0}) {
      print "C is defined";
   }
   print Dumper($foo);

}

abcde();
/tmp()$ perl foo.pl 
$VAR1 = {
          'a' => 1,
          'b' => 2
        };
$VAR1 = {
          'a' => 1,
          'b' => 2
        };
$VAR1 = {
          'a' => 1,
          'b' => 2
        };
$VAR1 = {
          'c' => {},
          'a' => 1,
          'b' => 2
        };

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