If you can then ditch the unsortable date format. Otherwise just transform the date before the sort and use the ISO format internally. Using non ISO compliant dates in computer software is inherently evil so doing this will be a net improvement (IMO) to your code. The sooner these regional date formats go the way of the steam engine the better. Something like the following regex should do it:

s!(\d+)/(\d+)/(\d+)!sprintf"%04d-%02d-%02d",$3,$2,$1!e;
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With this representation used internally you can use lexicographical sorting on the keys of the hash. Ie:

my @sorted=sort keys %hash;
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Alhough even here you are introducing year 10000 incompatibilities. If that worries you then you should use some other technique like the Swartzian Transform. Me, I tend to think that if they are still using my code 7000 years from now they deserve what they get.

my $lastdate;
foreach my $date
  # n.b. $a and $b interpreted specially in a sort block
  ( sort { sFormat( $a ) <=> sFormat( $b ) }
    keys %datecount
  ) {
    print "\n", $date, ": ", $datecount{$date};
    $lastdate = $date; # overwritten in order until last
}

sub sFormat { # convert dd/mm/yyyy into yyyymmdd
    my $old = shift;
    $old =~ /^(\d{2})\/(\d{2})\/(\d{4})$/
        or die "Programming error: Invalid format $old\n";
    return $3 . $2 . $1;
}
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If you want to go with your original approach, the following maybe useful:

my @dates = qw(5/7/2001 3/10/2005 1/1/1996 2/1/1993);

foreach my $date (@dates) {
  if ($date =~ m|(\d{1,2})/(\d{1,2})/(\d{4})|) {
    my $new_date = sprintf("%d/%02d/%02d", $3, $2, $1);
    ...
  }
}
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Hope this helps, -gjb-

Thanks for the suggestion gjb. Does that sort the hash too. I need to sort the dates, find the last date, and perform some calculations later in the script. Also please let me know what qw() is used for.

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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It returns a list of words without doing interpolation on them. It is just a convenient way of writing:

  my @dates = ('5/7/2001' '3/10/2005' '1/1/1996' '2/1/1993');
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Hope this helps, -gjb-

>Does that sort the hash too

You cannot sort a hash : a hash is by construction with no particular order. What you can do is sort the keys of a hash and print the corresponding values.

When you do  foreach (keys %hash) you get the keys in an order that is undefined although always the same. But it will change if you add or remove keys, therefore if you need a specific order you should do  foreach ( sort { my sort function} keys %hash )