I solved the problem

@dates = ('20130601', '20130401', '20130501');
my @sorted =
   map $_->[0],
   sort { $a->[1] cmp $b->[1] }
   map [ $_, join('', (/(..)(..)(....)/)[0,1,2]) ],
   @dates;  # DD-MM-YYYY
print "@sorted\n";
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Output

20130401 20130501 20130601
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Thanks all

Do you realize, that

join('', (/(..)(..)(....)/)[0,1,2])
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does not change a string $_ of length 8 in any way? (Apart from burning CPU cycles...). The regex also does not fit very well when splitting YYYYMMDD into pieces.

Yes, but the real strings that have to be sorted may not be that easy, containing e.g. rubbish at start, or some punctuation, or - worse - be of different, yet regexpable format. (I don't know for sure, just a thought).

..I solved the problem..
Good... but why not just use sort just as you have been told previously on this post.
Yes your regexes matched successfully, see

Read more... (4 kB)
but even at that it a lot of work.
However, if you must use "Schwartzian transform", you can sort on the month, since the year is the same like this:

use warnings;
use strict;

my @dates = ('20130601', '20130401', '20130501');

print join ' ' => map{$_->[0]}
              sort{$a->[1] <=> $b->[1]}
              map{/.{4}(.{2})/;[$_,$1]} @dates;
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But sincerely, for this use sort just like other monks told you.
If you tell me, I'll forget.
If you show me, I'll remember.
if you involve me, I'll understand.
--- Author unknown to me