in mySQL, you can make any one database the default database. however, you can simultaneously run a perl script which contains DBI commands which use another database to run operations on. (the command to switch the database is "use 'databasename'".

I cant imagine how the databases got deleted just by switching the default database. in mySQL, when a database is created, an operating system folder is created called with the database name,which is under the MySQL\data\ folder (for e.g. c:\mySQL\data\database1), which contains all the data files for that database. you might want to be careful about who has access to those folders. the culprit might not even need to log into mySQL to delete the database...all he (or she) needs to do is delete those operating system files which i just mentioned in order to cause havoc...