pg_restore in debian postgresql 7.3.2-3

I installed the above package on my debian box. The pg_restore command
fails with the following error:

pg_restore: [archiver (db)] could not execute query: ERROR:  stat failed on
file '$libdir/plpgsql': No such file or directory

my environment ist set to:

PGDATA=/var/lib/postgres/data
PGLIB=/usr/lib/postgresql/lib
POSTGRES_HOME=/var/lib/postgres
POSTGRES_LOG=/var/log/postgres.log

The plpgsql.so lib is in the $PGLIB directory.

Any hints?

Thanks & best regards
Herbie