Welcome to Linux Questions.
I wouldn't worry too much of that, subversion is already
installed in /usr , unless you said no to installing it.
See your package list :
'ls /var/log/packages > xxx.txt'
If it is about the 'svnlib*'s at all ???
Installing with cpan just would overwrite the already
installed 56 'svnlib*'s in /usr/lib/
The Slackware version is subversion-1.5.4 , the version
in Alien-SVN-22.214.171.124 is 1.4.6 .
Try if the application is working, then all is OK,
else ask again, there are other ways to install it all.
//ginstall: unrecognized option `--prefix=/usr'
Probably a bug in one of the perl build scripts. Make
or install usually don't have that option.
Good luck !