A few more things forgot to mention, about the return values: normally, if the structures are able to be compared, it will return -1, 0 or +1 meaning struct on left compared 'less', or equal or struct on rt.

The ordering of structures is definitely *arbitrary* or *subjective* -- I compare keys of hashes, for example, but whether or not that ordering is relevant or pertinent is entirely arbitrary -- as mentioned before, I really wanted to know if the structures were equal or not, but I figured, I might as well try *some* ordering and get some side benefit of possibly being able to sort data-structs into some order.

For hashes, I then pull up the values for the keys and sort those and compare those.

If you get back an 'undef', it means it couldn't compare it -- not that they were unequal(or equal).

Even if the underlying data structures are the same -- if they are blessed data structs in different classes, they return undef.

On return values, I use a 2 element array with the 2nd element being a number pointing at the test in the Cmp that failed. That helped me narrow down problems and such. They may or may not be useful.