The situation, as you've described it, is impossible. Both sqlplus and DBD::Oracle use the same interface to Oracle. Start looking for some differences in how you execute the commands.

• Do you have ORACLE_HOME defined the same in both circumstances?
• Are you sure you're using the same database name, user, password, etc., in both cases?
• Is your perl script running from the command line or a CGI? Is it the same user executing both scripts?

If none of these give you a clue, try using a third tool (tnsping, toad, tora, etc.) to get another data point.