The Perl backend to slapd(8) works by embedding a perl(1) interpreter into slapd(8). Any perl database section of the configuration file slapd.conf(5) must then specify what Perl module to use. Slapd then creates a new Perl object that handles all the requests for that particular instance of the backend.
The Shell backend to slapd(8) executes external programs to implement operations, and is designed to make it easy to tie an existing database to the slapd front-end. This backend is is primarily intended to be used in prototypes.
slapd-shell(5) and slapd-perl(5)