One of the prereq's for accessing an oracle server is an oracle client. DBI depends on a DBD to access any database. The DBD can use native code to access the database. In oracle's case, the oracle client is needed. It is almost never the case that a DBD will have the client implemented within it, in perl. Minimally, you need the oracle client for linux. To compile, I'm quite sure you need the development kit for oracle installed as well. They don't take up herendous amounts of space.

Update: Forgot to mention, ORACLE_HOME is the dir where you installed the oracle client... so it can find the drivers and the various ora config files, should they be needed.