#!/usr/bin/perl -w

use strict;

open (DATA, "./data.txt") or die $!;  # for example

my @sorted = map { $_->[1] }
             sort { $a->[0] cmp $b->[0] }
             map { [ &fix_date($_), $_ ] } <DATA>;

for (@sorted) { print $_ };

sub fix_date {
   my ($d,$m,$y) = ( shift =~ m|(\d\d)/(\d\d)/(\d{4})| );
   return "$y-$m-$d";
}