sort(3)

sort(3perl)            Perl Programmers Reference Guide            sort(3perl)

NAME
       sort - perl pragma to control sort() behaviour

SYNOPSIS
           use sort 'stable';          # guarantee stability
           use sort 'defaults';        # revert to default behavior
           no  sort 'stable';          # stability not important

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

DESCRIPTION
       With the "sort" pragma you can control the behaviour of the builtin
       "sort()" function.

       A stable sort means that for records that compare equal, the original
       input ordering is preserved.  Stability will matter only if elements
       that compare equal can be distinguished in some other way.  That means
       that simple numerical and lexical sorts do not profit from stability,
       since equal elements are indistinguishable.  However, with a comparison
       such as

          { substr($a, 0, 3) cmp substr($b, 0, 3) }

       stability might matter because elements that compare equal on the first
       3 characters may be distinguished based on subsequent characters.

       Whether sorting is stable by default is an accident of implementation
       that can change (and has changed) between Perl versions.  If stability
       is important, be sure to say so with a

         use sort 'stable';

       The "no sort" pragma doesn't forbid what follows, it just leaves the
       choice open.  Thus, after

         no sort 'stable';

       sorting may happen to be stable anyway.

CAVEATS
       As of Perl 5.10, this pragma is lexically scoped and takes effect at
       compile time. In earlier versions its effect was global and took effect
       at run-time; the documentation suggested using "eval()" to change the
       behaviour:

         { eval 'no sort "stable"';      # stability not wanted
           print sort::current . "\n";
           @a = sort @b;
           eval 'use sort "defaults"';   # clean up, for others
         }
         { eval 'use sort qw(defaults stable)';     # force stability
           print sort::current . "\n";
           @c = sort @d;
           eval 'use sort "defaults"';   # clean up, for others
         }

       Such code no longer has the desired effect, for two reasons.  Firstly,
       the use of "eval()" means that the sorting algorithm is not changed
       until runtime, by which time it's too late to have any effect.
       Secondly, "sort::current" is also called at run-time, when in fact the
       compile-time value of "sort::current" is the one that matters.

       So now this code would be written:

         { no sort "stable";      # stability not wanted
           my $current;
           BEGIN { $current = sort::current; }
           print "$current\n";
           @a = sort @b;
           # Pragmas go out of scope at the end of the block
         }
         { use sort qw(defaults stable);     # force stability
           my $current;
           BEGIN { $current = sort::current; }
           print "$current\n";
           @c = sort @d;
         }

perl v5.30.3                      2020-06-07                       sort(3perl)