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use re 'debug';

...
   map [ $_, join('', (/(..)(..)(....)/)[0,1,2]) ],
   @dates;  # DD-MM-YYYY
print "@sorted\n";

Compiling REx `(..)(..)(....)'
size 21 Got 172 bytes for offset annotations.
...
Match successful!
20130401 20130501 20130601
Freeing REx: `"(..)(..)(....)"'

use warnings;
use strict;
...
print join ' ' => map{$_->[0]}
              sort{$a->[1] <=> $b->[1]}
              map{/.{4}(.{2})/;[$_,$1]} @dates;