This part of the manual is the description of the client-side programming interfaces and support libraries for various languages.
pgobjectDBpgqueryobjectpglargeThe Perl interface (either the old Pg.pm or the newer and recommended DBD::Pg) should not be documented here -- neither package is shipped with PostgreSQL. The docs for Pg.pm should be available at the Pg.pm site, and DBD::Pg docs are available from CPAN, as well as the various sources for DBI documentation.
The same applies to other language interfaces not distributed with PostgreSQL.