Try tie'ing without O_CREAT:

tie %dictf, "DB_File", "f.db", O_RDWR, 0666, $DB_BTREE or die "Cannot 
+open file f.db: $!\n";
[download]

So, in hindsight, my unlink was unnecessary. And note that the default option is O_CREAT|O_RDWR, so you need to be explicit if you don't want to wipe it out.