Generally, sort functions are designed to do sorting as efficiently as possible, typically in O(n log(n)) comparisons. I'm not aware of any way to force perl's sort to do all n(n - 1) / 2 comparisons: if the purpose is not sorting, best not use sort() to do it.

The results from the comparator are intended to describe an ordering on the elements: compare(A, B) < 0 asserts that A < B, etc. There is an expectation of consistency here, so we expect identities like these to hold:

A = A
A = B  <=>  B = A
A < B  <=>  B > A
A <= B  <=>  B >= A
(A < B) & (B < C)  =>  A < C
(A <= B) & (B <= C)  =>  A <= C

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Note this important paragraph skulking near the end of perldoc -f sort:

            The comparison function is required to behave. If it retur
+ns
            inconsistent results (sometimes saying $x[1] is less than 
+$x[2]
            and sometimes saying the opposite, for example) the result
+s are
            not well-defined.

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For a bit more about what can go wrong, see the accepted response on this related C++ question: What will std::sort do if the comparison is inconsistent?. I'm not aware that perl's implementation is at risk of crashing, but you are certainly entitled to get weird results.

In reply to Re: sort of misunderstanding of sort by hv
in thread sort of misunderstanding of sort by Discipulus

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