Encrypt/decrypt strings. That is it. People can use Data::Dumper or Storable if they want to dump data structures. Allow it to handle a list in and out. If a list of strings are passed in in array context, send back a list of (en|de)crypted strings. If the output context is scalar, do a join on the data coming in. No need for more complexity, we already have interfaces for dealing with that. Text is the common denominator of both Unix and Perl. Perl is good at making things look like text.

Not worrying about the size of blocks is good. Hashing of user supplied keys is good.

Oh, and supply another pair of functions which take handles in and out. For encrypting a lot of data there is no need to have it all in memory at once.