How do I keep persistent data across program calls?

For some specific applications, you can use one of the DBM modules. See AnyDBM_File. More generically, you should consult the FreezeThaw or Storable modules from CPAN. Starting from Perl 5.8 Storable is part of the standard distribution. Here's one example using Storable's store and retrieve functions:

    use Storable; 
    store(\%hash, "filename");

    # later on...  
    $href = retrieve("filename");        # by ref
    %hash = %{ retrieve("filename") };   # direct to hash