tied interface to persistent file

 Tie::Persistent makes working with persistent data real easy by using the tie
 interface. It works by storing data contained in a variable into a file (not
 unlike a database). The primary advantage is speed, as the whole datastructure
 is kept in memory (which is also a limitation), and, of course, that you can
 use arbitrary data structures inside the variable (unlike DB_File). Note that
 it is most useful if the data structure fits into memory.