This doesn't sort the hash, it only converts the date from D/M/YYYY to YYYY/MM/DD so that it can be used as a key in your %datecount hash. This hash can be sorted the way you do it in your code fragment.

As to qw:

$ perldoc -f qw
    qw/STRING/
      Generalized quotes.  See "Regexp Quote-Like Operators"
      in perlop.
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  my @dates = ('5/7/2001' '3/10/2005' '1/1/1996' '2/1/1993');
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Hope this helps, -gjb-