The following modules make it easier to handle the authentication phase:

AuthenCache     Cache authentication credentials
AuthCookie      Authentication and authorization via cookies
AuthDigest      Authentication and authorization via digest scheme
AuthenDBI       Authenticate via Perl's DBI
AuthenIMAP      Authentication via an IMAP server
AuthenPasswdSrv External authentication server
AuthenPasswd    Authenticate against /etc/passwd
AuthLDAP        LDAP authentication module
AuthPerLDAP     LDAP authentication module (PerLDAP)
AuthenNIS       NIS authentication
AuthNISPlus     NIS Plus authentication/authorization
AuthenSmb       Authenticate against an NT server
AuthenURL       Authenticate via another URL
DBILogin        Authenticate to backend database
PHLogin         Authenticate via a PH database

All available from CPAN. See the module manpages for more information.