Thanks to Peter Eisch, an apache conf file :

  # with apache < 2.1.5 use just 'Listen 0.0.0.0:25'
  Listen 0.0.0.0:25 smtp
  AcceptFilter smtp none
  ## "smtp" and the AcceptFilter are required for Linux, FreeBSD 
  ## with apache >= 2.1.5, for others it doesn't hurt. See also
  ## http://httpd.apache.org/docs/2.2/mod/core.html#acceptfilter
  ## and http://httpd.apache.org/docs/2.2/mod/mpm_common.html#listen
  HostnameLookups On

  LoadModule perl_module lib/httpd/mod_perl.so

  <Perl>
    use lib qw( /home/smtpd/qpsmtpd/lib );
    use Apache::Qpsmtpd;
  </Perl>

  <VirtualHost _default_:25>
    PerlSetVar QpsmtpdDir /home/smtpd/qpsmtpd
    PerlModule Apache::Qpsmtpd
    PerlProcessConnectionHandler Apache::Qpsmtpd
  </VirtualHost>

Don't forget to explicitly set the plugin dirs config:

  $ echo "/home/smtpd/qpsmtpd/plugins" >> /home/smtpd/qpsmtpd/config/plugin_dirs

or you'll get an error like

[Mon Jul 30 20:50:30 2007] [error] Can't locate object method "new" via package "Qpsmtpd::Plugin::logging::warn" (perhaps you forgot to load "Qpsmtpd::Plugin::logging::warn"?) at /home/smtpd/qpsmtpd/lib/Qpsmtpd.pm line 326.\n

(…since rev 762 fixed in trunk to give a better error message)