...->{...}
...->[...]
[download]

is really equivalent to

( ... //= {} )->{...}
( ... //= [] )->[...]
[download]

when they're found in an lvalue context (such as the argument of a function like defined).

You can disable this behaviour for the arguments of defined and exists by adding

no autovivification;
[download]

Ref: autovivification