If for some reason, you can not use a database (which although the program specification may not call for shared addressbooks and stuff like that, trust me, it will change, and you will need a database... program specifications do that...) then my suggestion is as follows:

My suggestion, is that you use the CPAN module MLDBM. What this module alows you to do is store complex data structures within a DBM without you having to deal with all of the complexities of serialization and unserialization. Where I in your situation I would do something like, each entry into the addressbook is a hash:

my $entry = { NAME    => "eduardo arino",
              PHONE    => "(555) 555-1212",
              ADDRESS    => "666 Dante Circle, Murfreesboro, TN", 
              EMAIL    => "ed@NO_SMAP.home.com" };
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and then, index the hash that is the tied MLDBM DB file by whatever it is you wanted, name, phone, address, whatever. Being that you could access it like a hash, your data manipulation problems would be greatly minimized, and being that this is not a speed critical piece of code, the overhead in the MLDBM package would probably not kill you...

then again, there might be some other way of doign it... it's just that whenever i hear people talk of using flat files, I get this feeling that it's going to end up coming back to haunt them... I know the few times i have, it did...