Why is broccoli not at the end of the list? I know there's no entry for 9, but why is it inserted there?

Because the lookup of 9 in %ranking returns undef, and undef (issues a warning and) acts as 0 in a numerical comparison. Hence broccoli gets sorted as if its ranking was 0.

If you want unranked foods to sort to the end of the list, you could do something like this:

print %{$_} for sort {  ( $ranking{$a->{number}} // 1e308 ) <=> ( $ran
+king{ $b->{number}} // 1e308 ) } @food;;
number 1 name apple
number 2 name banana
number 5 name orange
number 10 name grape
number 9 name broccoli
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Note: the parens are required.