This line:

my @arrays = ([2,4,6],[8,9,5],[1,2,3]);

creates an array of references, like this:

@arrays = ($ref1, $ref2, $ref3);

Your array access here:

print $arrays[1][2];

is shorthand for:

$arrays[1]->[2]
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which is shorthand for:

@{$arrays[1]} [2]
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Because the elements of @arrays are references, in order for you to get the second reference, you would write this:

$arrays[1]
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And to convert a reference to the actual array (i.e. dereference the reference), you would write this:

@{$arrays[1]}
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Braces are used to turn a reference into the thing it references.