The DBI module enables your Perl applications to access multiple database types transparently. You can connect to MySQL, MSSQL, Oracle, Informix, Sybase, ODBC etc. without having to know the different underlying interfaces of each. The API defined by DBI will work on all these database types and many more.
You can connect to multiple databases of different types at the same time and easily move data between them. The DBI layer allows you to do this simply and powerfully. Here's a diagram that demonstrates the principle: