failed: ORA-24327: need explicit attach before authenticating a user (DBD ERROR: OCISessionBegin) at

If you are using Oracle and your web perl application is getting the following error:

failed: ORA-24327: need explicit attach before authenticating a user (DBD ERROR: OCISessionBegin) at

the issue could be you have change the ORACLE_SID in listener.ora and tnsnames.ora

Please restart your listener to fix the issue.