I don't really understand why I need use @{} here to dereference, can somebody explain it for more details?
By using the square brackets, you have created a reference to an array. Since you want to access the elements of the array reference, you first need to de-reference the array. One way to do so is to use the @{} syntax. Here are some good resources on the topic:

In reply to Re^3: how to store an array to a pseudohash? by toolic
in thread how to store an array to a pseudohash? by Hanken

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