Assuming you have enough disk space, you can use DB_File or Tie::Hash::DBD:

use DB_File;
tie my %hash, "DB_File", "file.db", O_RDWR|O_CREAT, 0666;
# now use hash as normal
[download]

use Tie::Hash::DBD;
tie my %hash, "Tie::Hash::DBD", "dbi:SQLite:dbname=db.tie";
# now use hash as normal
[download]

Read the documentation of your prefered module to see what the options and possibilities, like data persistence and nested structures, are.