use DB_File::Lock;
tie(%db_hash, 'DB_File::Lock', "/path/to/my/database",  O_RDWR, 0600, $DB_HASH, 'write') or die;
1 while $db_hash{my $num = get_random_number()};
$db_hash{$num}++;
  ... do something with $num ...
untie(%db_hash);