You're being burnt by a combination of operator precedence and context.

my @index = @{$args{Index}} || ();
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is interpreted as

my @index = (@{$args{Index}} || ());
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Then the || operator forces scalar context on @{$args{Index}} which therefore gives the number of elements in the array (which is 2).

On the other hand,

my @index = @{$args{Index}} or ();
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is interpreted as

(my @index = @{$args{Index}}) or ();
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So the array is evaluated in list context and everything works as expected.