If the assignment doesn't allow you to use a module, you might start with dbmopen, which will let you tie a hash variable to a file, and then automatically update the file as you make changes to the hash:

#!/usr/bin/env perl
use 5.010; use strict; use warnings;

dbmopen(my %h, './database.dbm',0666) or die $!;  # open database file
$h{mykey} = 'my value';                           # save a key/value p
+air
dbmclose(%h);                                     # close database

dbmopen(my %i, './database.dbm',0666) or die $!;  # reopen database fi
+le
say "$_ : $i{$_}" for keys %i;                    # print key/value pa
+irs
dbmclose(%i);
[download]

When you're comfortable with that, look into tie, which is meant to supersede the dbm* routines and gives you a more flexible way to do the same sort of thing (I think there may be portability issues with dbmopen, which tie was created to alleviate). If you want a database file that's human-readable, you can use something like YAML or JSON on the backend, but those would require a lot of coding or a module.