+=item B<RegisterLegacyFlush> I<true|false>
+
+The C<Perl plugin> used to register one flush callback (called B<"perl">) and
+call all Perl-based flush handlers when this callback was called. Newer versions
+of the plugin wrap the Perl flush handlers and register them directly with the
+daemon I<in addition> to the legacy B<"perl"> callback. This allows to call
+specific Perl flush handlers, but has the downside that flushing I<all> plugins
+now calls the Perl flush handlers twice (once directly and once via the legacy
+callback). Unfortunately, removing the B<"perl"> callback would break backwards
+compatibility.
+
+This option allows you to disable the legacy B<"perl"> flush callback if you care
+about the double call and don't call the B<"perl"> callback in your setup.
+