DBI connects to one oracle instance but cannot connect to others

We run Oracle 10g under Linux. There are seven database instances:
prod, tmp1,...,tmp6. The TNSNAMES.ORA file treats all seven instances
equally:

prod =
(DESCRIPTION=
(ADDRESS_LIST=
(ADDRESS=
(COMMUNITY = tcp.world)
(PROTOCOL=tcp)
(HOST=localhost)
(PORT=1521)
)
)
(CONNECT_DATA =
(SERVER = DEDICATED)
(SID = prod)
)
)

..... identical segments for the tmp1 - tmp6 instances

I can sqlplus into any instance. I can run a local perl script to
connect to the production instance "prod". I can also run perl
scripts from a remote server to connect remotely to any of these
instances. But my perl script on localhost cannot connect to any of
the tmp instances tmp1...tmp6.

This script works:

use DBI;

$ENV{"ORACLE_HOME"} = "/u01/app/oracle/OraHome_1";
$ENV{"ORACLE_SID"} = "prod";

$odsn="DBI:Oracle:prod";
$ouid="xxxx";
$opwd="xxxx";

$odbh= DBI->connect ($odsn,$ouid,$opwd);

--- works perfectly BUT the identical for the tmp instances DOES NOT
WORK:

use DBI;

$ENV{"ORACLE_HOME"} = "/u01/app/oracle/OraHome_1";
$ENV{"ORACLE_SID"} = "tmp1";

$odsn="DBI:Oracle:tmp1";
$ouid="xxxx";
$opwd="xxxx";

$odbh= DBI->connect ($odsn,$ouid,$opwd);

I get the following error message: TNS:could not resolve the connect
identifier specified

What am I doing wrong????

Thank you for any hint.
Anke

.

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