Assume you have a vector A and you going to sort it according to a function f(x). If you sort it naively, whenever you are going to compare two items, you call the function. Because the best sorting algorithm we know nowadays is $O(n \log n)$, you call such function a lot.

Schwartzian transformation is, simply speaking, create a 2D array of (key,value) pair by running the function f(x) against each item. Then do the sorting and remove the key after finished. This makes less call to f(x).

Seems trivial, but what is beautiful about Schwartzian transform is in perl:

@sorted =
map  { $_->[0] } sort {$a->[1] <=> $b->[1] } map { [$_, f(\$_)] }
@unsorted;


All these can be done in one line, just as the sort call.