Why can't I say: print "@${a}\n";

I don't know. You should be able to, and I can.

$ perl -wE'$a = [1,2,3]; say "@${a}";'
1 2 3
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What do you get? Perhaps $a didn't contain a reference to an array when you tried it?

Also, why the curly braces surrounding the array (@{..})

In general, they're optional. In @{ $h{a} }, they override precendence.

@$h{a}
=> @{$h}{a}
=> A hash slice that expects $h to be a reference to a hash.

@{ $h{a} }
=> @{$h}{a}
=> An array dereference that expects $h{a} to return an array ref.
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