src/subversion contains the source of the latest SubVersion. But that is most often not helpful.
The user needs SVN::* modules matching his svn command version, which what is sure to be compatible with the local working copies.
When the SVN::* don't match the svn command, the perl modules often don't work with the user working copy as the SVN::* modules have a more recent version of SubVersion that requires upgrading the working copy.