$record[$_]->{bytes} = [get_bytes($num)]->[$_] for 0..$num;
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but you'll better not use this, because get_bytes will be invoked $num times! And this is really ineffective.

addition:
but your three lines of code next to @tmp assignment better written as:

$record[$_]->{bytes} = $tmp[$_] for 0..$#tmp;
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Courage, the Cowardly Dog