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my $href;
$href->{smith}{john}{eyecolor} = 'blue';

my $href;
my $color = $href->{smith}{john}{eyecolor};

my $href;
if (exists $href->{smith}{john}{eyecolor}) {
   # do something
}

my $aref = undef;
my $a = $$aref[0];

my $aref = undef;
my @a = map {$_} @$aref;