Is there a practical reason for the 2nd example?

Orthogonality. Without the curly braces @$a[1] is unintuatively equivalent to @{$a}[1]:

$a = [ [1],[2],[3] ];;

print @$a[1];;
ARRAY(0x1b7ae0)

print @{$a}[1];;
ARRAY(0x1b7ae0)
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Instead of:

print @{ $a[1] };;
2
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You can get away with not using the curlies for a single depth dereference, but it is required for greater depths.

And this is an error:

print @$a->[1];;
Using an array as a reference is deprecated at (eval 15) line 1, <STDI
+N> line 7.
ARRAY(0x1b7ae0)
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That needs to be this:

print @{ $a->[1] };;
2
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I find it easier to always use the curlies rather than have to think about the exceptions.