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;
}