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:

• References quick reference
• perlreftut
• perldsc
• perllol