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: