I don't like the comma-prepending style. You can easily add a new line at the bottom that way, but not at the top. It might be fine for languages that are inflexible about their commata, but Perl is not - you can append one to the last value with no harm. My formatting looks like this:

my %hash = (
    one   => 1,
    two   => 2,
    three => 3,
);
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Note the "errant" comma after the last value. Separating the assignment and closing bracket on their own lines adds more convenience. This way, I can add a hash key anywhere - even at the bottom or the top - without having to edit any other line than the one I'm on.

Makeshifts last the longest.