Sure, just after I post the message I realize what I did. It should have been.

sub unpacked {
        my($a_num) = unpack( 'V', substr( $a, 0, 4 ));
        my($b_num) = unpack( 'V', substr( $b, 0, 4 ));
        return $a_num <=> $b_num;
};
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