I just faced the same problem. With a few lines, I moved my ~4GB hash from RAM to disc:

use DB_File;
my $db     = '/path/to/hash.db';
my %hash;
tie (%hash, "DB_File", $db, $DB_BTREE) or die("Unable to tie $db: $!")
+;
# do things with hash
untie %hash;
[download]

Note that $DB_BTREE format is a lot faster than the default, $DB_HASH. (Twice as fast, in my benchmarks.) See also 152966, and the thread leading up to it.