The Benchmark module is a big help with this sort of thing.
What the Schwartzian Transform does is reduce the overall number of calls to the function that computes the value of your item. Your sort is going to do roughly (n * log n)
comparisons where n is the number of items in your list. If the function that computes the value of the list item is significantly more expensive than a simple comparison you want to reduce the number of calls to that more expensive function.
The map at the end of the Schwartzian transform only runs the function once for each element of the list. Of course you incur some overhead in building an arrayref, and looping again at the end, but for lists with many items reducing the number of computations can save a lot of cycles. You'll see very different results based on how expensive it is to compute the sort value.