Let's take a list of files and use a Schwartzian Transform to sort it by file size descending, then alphabetical order ascending. 
my @sorted_files
  = map {$_->[0]}
    sort {$b->[1] <=> $a->[1] or $a->[0] cmp $b->[0]}
    map {[$_, -s]} @files;

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Here is the same code written as a normal sort. 
my @sorted_files = sort {-s $b <=> -s $a or $a cmp $b} @files;

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Clearly the Schwartzian Transform is more complex. But if you have a list of 1000 files, it's also about 10 times faster. Which is why we learn it.

Now to explain my Ruby comment. In Ruby, arrays have a sort_by method. So in this example you'd write: 

sorted_files = files.sort_by {|f| [- test(?s, f), f]};

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and you've written the more efficient sort with less code than the regular sort. This does not work in Perl first of all because we don't have a sort_by method, and furthermore because Perl doesn't do anything useful when you try to sort array references. (Ruby sorts them lexicographically, with each field sorting in its "natural" way.)

In reply to Re^5: top ten things every Perl hacker should know by tilly
in thread top ten things every Perl hacker should know by apotheon

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