export ORACLE_HOME=/local/src/local.perl/oracle/instantclient_10

export LD_LIBRARY_PATH=$ORACLE_HOME/lib
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Sometimes it needs to be /lib32 or /lib64 depending on how Oracle is installed, but then again that probably doesn't apply to the instant client. I haven't tried this for Oracle yet, but for Sybase, I added a block to Sybase.pm and forced it to get it right without user environment variables.

{
    package DBD::Sybase;
    local $ENV{SYBASE};
    $ENV{SYBASE} = "/syb/sybase00/rel12.5";

    use DBI ();
    use DynaLoader ();
    use Exporter ();
...

}
1;
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There is really no downside to hard-coding the environment variables if you don't plan to move the client directory around. If you upgrade it, DBD::Oracle has to be relinked anyways.