Thank you for your reply ! I apologize in advance for the French in the error messages below (all this is happening on a French Windows XP box).

First of all, the TWO_TASK environment variable is not set in this Windows system. I have not tried to use it. Here is the output of the snippet that you suggested: 
C:\test>perl oracle_test.pl
DBI connect('sid=JUPITER;host=jupitercluster','user',...) failed: ORA-
+12505: TNS : le module d'écoute (listener) n'a pas pu résoudre le SID
+ donné dans le descripteur de connexion (DBD ERROR: OCIServerAttach) 
+at oracle_test.pl line 11

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As for tnsping, I guess that it gives a strong indication. Using it produces a TNS-03505 error for every single alias in the tnsnames.ora file ! This is strange because I can successfully use all the other tnsnames.ora aliases from within Perl/DBI, as well as using the Oracle SQL Developer client. I mean, except the one I am having a problem with, which does not work from within Perl/DBI, but *does* work using the Oracle SQL Developer client. 
c:\test> tnsping JUPITER
   or 
c:\test> tnsping jupiter
TNS Ping Utility for 32-bit Windows: Version 9.2.0.1.0 - Production on
+ 13-MAI-2013 17:17:40

Copyright (c) 1997 Oracle Corporation.  All rights reserved.

Fichiers de paramètres utilisés :
C:\Oracle\network\admin\sqlnet.ora

TNS-03505: Echec de la résolution du nom

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Here is the (short) content of sqlnet.ora: 
# Generated by our DBA

NAMES.DEFAULT_DOMAIN = mycompany.com

NAMES.DIRECTORY_PATH= (TNSNAMES)

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Any further suggestions ? Again, thanks a lot for your answer. And yes, learning about Oracle and Perl/Oracle is fun ;-)

In reply to Re^2: DBI and Oracle ORA-12505 error by jmb
in thread DBI and Oracle ORA-12505 error by jmb

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