my @newfiles = ("new_one.txt",
                "old_one.txt",
                "old_two.txt",
                "new_two.txt");
my %oldfiles;
   $oldfiles{"old_one.txt"}="this is the old one";
   $oldfiles{"old_two.txt"}="this is the old two";
   $oldfiles{"old_three.txt"}="this one should be deleted";
   $oldfiles{"new_two.txt"}="this one has been added already";

my %newfiles;

foreach (@newfiles){
        $newfiles{$_}++;
        next if exists $oldfiles{$_};
        print "have to add $_\n";
}

foreach (keys %oldfiles){
        next if exists $newfiles{$_};
        print "have to remove $_\n";
}