You're most welcome!

Yes, Komodo appears to just be alerting you about <DATA>, but should certainly know better, since there's a __DATA__ section.

Try the following:

use strict;
use warnings;
use File::stat;

chomp( my @fileNames = <*.bak> );

my @sortedFileNames = map $_->[0], 
    sort { $b->[1] <=> $a->[1] }
    map { my $stat = stat $_; [ $_, $stat->mtime ] }
    grep /^backup_\d\d_\d\d_\d{4}.bak$/, @fileNames;

shift @sortedFileNames;

if (@sortedFileNames) {
    print "$_\n" for @sortedFileNames;
    #unlink @sortedFileNames;
}
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This stats each file for the modification time, using it in the sort. Also, note that a file glob's used to read directory files...