You can't print "@${a}\n" because you don't have an array reference named 'a'. This works:

my $a = \@{$h{a}};
print "@${a}\n"; # or just "@$a\n";
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And the outer curlies in "@{$h{a}}\n" surround an array reference. Reading it inside-out, you have a hash key $h{a} whose value is an anonymous array. To print that value, you dereference it with @{}.