|<- Scope of DBI ->|
                  .-.   .--------------.   .-------------.
  .-------.       | |---| XYZ Driver   |---| XYZ Engine  |
  | Perl  |       | |   `--------------'   `-------------'
  | script|  |A|  |D|   .--------------.   .-------------.
  | using |--|P|--|B|---|Oracle Driver |---|Oracle Engine|
  | DBI   |  |I|  |I|   `--------------'   `-------------'
  | API   |       | |...
  |methods|       | |... Other drivers
  `-------'       | |...

Taken from the DBI documentation.
Any number of databases at the same time.
Probably every RDBMS can be accessed with its own DBD.

While in many areas Perl has plenty of ways to achieve things in the area of database access a single package has evolved as the de-facto standard way of the low level access. The DBI - Database Independent Interface of Perl - is the module that most of the perl applications in the world use as their core. Between this module and between virtually and relational database we have a Database Driver - a module in the DBD::* namespace. For example there are modules such as DBD::Oracle and DBD::MySQL or DBD::Pg for PostgreSQL. Above DBI there are hundreds of higher level abstractions. Most of them live in the DBIx::* namespace on CPAN.