in reply to Is this ancient perl or just a cargo cult?
To me, this looks like a dodgy cargo cult way of doing an ordered hash.
It is neither dogdy; nor cargo-cult; nor do they pre-date hashes. Both became available at exactly the same time with the advent of blessed references circa. 5.001.
The code is using a blessed array rather than a blessed hash as the basis of its objects. This is more compact and faster than the "normal" blessed hashes.
The constants give programmer-friendly names to the fields; whilst still allowing the field lookups to be optimally efficient.
For completeness, field lookup with a hash still requires an array lookup; but first the key (field name) has to be translated into an index number. And then, if you happen to be unlucky, there might need to be a linear search as well.
blessed arrays with named constant field indexes are a less used, but perfectly valid, highly functional and more efficient way of implementing objects in Perl.