issues installing DBD-Pg-2.16.0 on linux-64bits

I'm having issue installing DB-Pg on OS: linux -64 bits, RHES5; PG vers
8.3.6 already running, DBI-1.609 installed : All this have been
installed as non root user .
DBI compiled with PREFIX='my_local_dir'( aka
PREFIX=/var/lib/pgsql/DBI-1.609)
now I'm trying to install DBD-Pg:
which pg_config
/opt/pgsql/bin/pg_config

perl Makefile.PL PREFIX=/var/lib/pgsql/DBD-Pg -I/var/lib/pgsql/DBI-1.609
Configuring DBD::Pg 2.16.0
PostgreSQL version: 80306 (default port: 5432)
POSTGRES_HOME: /opt/pgsql/
POSTGRES_INCLUDE: /opt/pgsql/include
POSTGRES_LIB: /opt/pgsql/lib
OS: linux
Could not load DBI::DBD - is the DBI module installed?