Perl in PostgreSQL is designed to allow you access to perl functions directly from SQL. While the SQL still looks like SQL, you can implement the function in Perl instead of say, PL/pgSQL. If you're smart about not crossing the RDBMS/DBI line too many times then this can be a definite boon - if you're not up on all the costs associated with moving things between your RDBMS and your external language then it may not look like its worth much.

For myself, I've never tried the perl extension because PL/pgSQL comes pre-built and I can get my work done with that.