Actually, the reason why your non-working examples "do not work" is simply because that @$hr{a} is ambigious: does it mean that $hr is an array reference or does it mean that the key 'a' from the hash reference $hr contains an array reference. If the case is the latter, you really should be using syntax like so:

@{$hr->{a}} = [qw(foo bar baz)];

@{$ar->[0]} = [0 .. 9];
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Update: thanks for the typo correction, ikegami.

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)