Or you could actually use all.

all { $res[$_ - 1]{end_date} eq $res[$_]{start_date} } 1 .. $#res
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Or if you want to know which ones are "bad" so you can output a message for each of them:

map { $res[$_ - 1] }
   grep { $res[$_ - 1]{end_date} ne $res[$_]{start_date} }
      1..$#res
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Exactly right, the problem is broken down via reduce and then some characterization. Ordering can be ascending/descending, strict/non-strict, involve numeric/string compare, etc.

However, the example given is a test for gaps or discontinuities rather than ordering. This, and the other topic relating to Testing, makes me think the concept OP is looking for might be invariant.

In fact the "concept OP is looking for" is called dependent type theory. However, this isn't even remotely implementable in Perl. Possibly in Scala or Haskell, definitely in more advanced languages like Idris. Not sure about Perl6.

So in the absence of "you cannot do it in a way that breaks invariants" types, a mixture of runtime/unit tests that can be fine-tuned on accuracy<------>speed scale makes for a poor but workable substitute.

In my opinion, anyway, but I'm going to put it to the test the hard way.

Why not OO framework then? Getters/setters, methods all in one place with internal logic and consistency checks as required. What problem is this that cannot be abstracted and subdivided into parts?