> Is this [autovivification] always good? No.

Good thing then that the programmer has the choice to use it or not.

    repl>>> no autovivification 'store'
    repl>>> my %h
    %h = ()
    repl>>> $h{'foo'}[3]{'bar'} = 'bletch'
    Runtime error: Can't vivify reference at (eval …)

> Is this [returning undef for non-existing keys/indexes] always good. […] the answer is no.

Good thing then that the programmer has the choice to use it or not.

    repl>>> use Hash::Util 'lock_keys'
    repl>>> my %h
    %h = ()
    repl>>> lock_keys %h
    ()
    repl>>> $h{no_such_key}
    Runtime error: Attempt to access disallowed key 'no_such_key' in a restricted hash at (eval …)

    repl>>> package Array::Bounded {
                use Tie::Array;
                use base 'Tie::StdArray';
                sub FETCH {
                    my ($self, $index) = @_;
                    die "Attempt to access out of bounds index $index in a restricted array" if $index >= $self->@*;
                    return $self->[$index];
                }
            }
    repl>>> tie my @a, 'Array::Bounded'
    repl>>> @a = ('a'..'f')
    @a = ('a'..'f')
    repl>>> $a[5]
    f
    repl>>> $a[6]
    Runtime error: Attempt to access out of bounds index 6 in a restricted array at (eval …)