Add back mod_perl to debian build

The build issues blocking its return appear to have been resolved.

debian/bootstrap.sh

@@ -28,7 +28,6 @@ avoid_mods=(
   endpoints/mod_reference
   endpoints/mod_unicall
   languages/mod_managed
-  languages/mod_perl
   sdk/autotools
   xml_int/mod_xml_ldap
   xml_int/mod_xml_radius