tie my %hash, 'Tie::Hash::Vivify', sub { 10 };
say $hash{foo};  # says 10
my $is_this_true = exists $hash{foo};

##</code><code>##

tie my %hash, 'Hash::DefaultValue', sub { 10 };
say $hash{foo};
my $is_this_true = exists $hash{foo};

##</code><code>##

tie my %hash, 'Hash::DefaultValue', sub { uc };
say $hash{foo}; # says "FOO"