You can't pass an array to a function.
If @a has 0 elements, func(@a) is the same as func().
If @a has 1 element, func(@a) is the same as func($a[0]).
If @a has 2 elements, func(@a) is the same as func($a[0], $a[1]).
If @a has 3 elements, func(@a) is the same as func($a[0], $a[1], $a[2]).
etc.

That's why the text speaks of an "array or hash element".

sub test1 { $a = $_[0]; }
sub test2 { $_[0] = $a; }

my @a;
test1($a[0]);  print(scalar(@a), "\n");  # 0
test2($a[0]);  print(scalar(@a), "\n");  # 1
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Update: Ah shoot! It had written $_[x] where I had meant to write $a[x]. Fixed. Thanks ysth and graff.