I really really wonder how you can think that

    func(
        aa => (my $aa = foo()),
        bb => (my $bb = bar()),
    );
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can be prettier than

    func(
        aa => scalar foo(),
        bb => scalar bar(),
    );
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One can argue code aesthetics all day, but your assignment can be a real pain in the long run and all it does is to fool the maintenance programmer into thinking that you especially want those values later. If you do this, although please don't, at least use a do block to handle the scoping issue (and make the maintenance programmer scratch his head a little less).

    func(
        aa => do { my $t = foo() },
        bb => do { my $t = bar() },
    );
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ihb

Read argumentation in its context!