I think I'm staying with is_sorted for simple checks like $a < $b, plus a separate subtest-based function for more complex conditions, for which I don't have an established name yet:

reduce_ok {
    is $a->{end_date}, $b->{start_date}, "reservations are coupled";
    ok !$b->{is_arrival}, "not expecting >1 arrivals";
} \@reservations;
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The current implementation of is_sorted will work like that, too, but that's a coincidence and should not be abused.

And happy new 2018!