well i didn't solve the problem, but i found a workaround. basically i remove the offending options and the include file. the following instructions work for dbd::oracle 1.19 on a gentoo-amd64 installation.

1) install the oracle libraries

strictly speaking you only need dev-db/oracle-instantclient-basic for dbd::oracle, but i always like to have sql*plus lying around, which requires the basic package, so i jut install sql*plus.

emerge dev-db/oracle-instantclient-sqlplus which also pulls in dev-db/oracle-instantclient-basic. these packages are fetch restricted so you will be required to follow the download instructions. following these instructions, you should have retrieved these packages:

instantclient-basic-linux-x86-64-10.2.0.3-20070103.zip
instantclient-sdk-linux-x86-64-10.2.0.3-20070103.zip
instantclient-sqlplus-linux-x86-64-10.2.0.3-20070103.zip

now move them into the /usr/portage/distfiles directory.

you should now be able to emerge dev-db/oracle-instantclient-sqlplus.

2) install DBD::Oracle

issue the command:

perl -MCPAN -e'install DBD::Oracle'

this fails with the following error:

x86_64-pc-linux-gnu-gcc: unrecognized option '-wchar-stdc++'
x86_64-pc-linux-gnu-gcc: unrecognized option '-cxxlib-gcc'
cc1: error: /ee/dev/bastring.h: No such file or directory

find the offending files in your cpan directory:

{~/.cpan/build/DBD-Oracle-1.19} grep -lr cxxlib *
Makefile
blib/arch/auto/DBD/Oracle/mk.pm
mk.pm

edit these files and remove the two invalid options and the include of bastring.h.
now build the module:

perl Makefile.PL -l
make
# make test generates lots of errors
make test
make install

you should now be ready to run.