use strict;
use warnings;

chomp( my @fileNames = <DATA> );

my @sortedFileNames = map $_->[0], 
	sort { $b->[1] <=> $a->[1] }
	map { my ( $d, $m, $y ) = /(\d+)/g; [ $_, "$y$m$d" ] }
	grep /^backup_\d\d_\d\d_\d{4}.bak$/, @fileNames;

shift @sortedFileNames;

if (@sortedFileNames) {
	print "$_\n" for @sortedFileNames;
	#unlink @sortedFileNames;
}

__DATA__
backup_21_01_2013.bak
file.txt
backup_20_01_2013.bak
what_is_this.doc
backup_24_01_2013.bak
never_open_this.docx
backup_22_01_2013.bak
stuff.ini
backup_23_01_2013.bak
more_stuff.ini
deleteOldBackups.pl