The documentation of Class::Struct says that $a->b in this case returns an array reference, so you can do:

@{$a->b} = @q;
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and

@x = @{$a->b};
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see documetation like perlreftut, perllol, and perlref for more details

Update: just wanted to note you should realize that these statements really copy the elements from one array to the other. It's often more efficient to pass array references around instead; see the docs I linked to.