I have written pre-forked server applications that have many children reading and writing to a flat-file database using file locking and it can add ten of thousands of entries a day on a poky Pentium III laptop. So I don't think the file locking will be a bottleneck.

The encryption/decryption cycling may be a little slow, depending on the size of your database. One alternative is to write each entry as its own encrypted little file, and write another perl program to read all these little files in at the end of the day to produce a big encrypted file.