If I'm understanding you correctly, perhaps the following will be helpful:

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

[download]

Output (the files that would be deleted):

backup_23_01_2013.bak
backup_22_01_2013.bak
backup_21_01_2013.bak
backup_20_01_2013.bak

[download]

If you populate @fileNames with the file names in the directory where the backups live, it will grep them only allowing backup-patterned files through. Then, using a Schwartzian transform, it sorts the backup file names in decending order and shifts off the first element (most recent backup file name) from @sortedFileNames. As it is now, the file names in @sortedFileNames are printed, but the unlink line can be uncommented, so all but the most recent backup files are deleted.

** Please thoroughly test and verify this on a copy of the backup directory before going live with it. **

In reply to Re: Delete all but the most recent backup file by Kenosis
in thread Delete all but the most recent backup file by jagexCoder

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