Programming Perl, 4th Edition

sort

Before v5.8, quicksort was the default algorithm for Perl’s built-in sort function. The quicksort algorithm has at its worst quadratic behavior, and it doesn’t necessarily preserve the order of elements that sort the same (so it’s unstable). Perl v5.8 changed the default to a merge sort, which at its worst has O(n·log n) behavior and preserves the order of equal elements (so it’s stable).

The sort pragma lets you choose which algorithm to use. And in case the default might someday change from a mergesort, you can choose a stable sort without picking the particular algorithm:

use sort "stable";          # guarantee stability
use sort "_quicksort";      # use a quicksort algorithm
use sort "_mergesort";      # use a mergesort algorithm
use sort "defaults";        # revert to default behavior
no  sort "stable";          # stability not important

use sort "_qsort";          # alias for quicksort

my $current;
BEGIN {
   $current = sort::current();     # identify prevailing algorithm
}