Access to the database itself from your Perl function can be done via an experimental module DBD::PgSPI (also available at CPAN mirror sites). This module makes available a DBI-compliant database-handle named $pg_dbh that can be used to perform queries with normal DBI syntax.
PL/Perl itself presently provides only one additional Perl command:
elog level, msgEmit a log or error message. Possible levels are DEBUG, LOG, INFO, NOTICE, WARNING, and ERROR. ERROR raises an error condition: further execution of the function is abandoned, and the current transaction is aborted.