There may be some reasons to keep older versions of perl lying around (older code that uses it, other libs compiled against it, etc).

But I can definitely think of one reason to have more than one instance of perl on the same system even if they are of the same version. If you are trying to run mod_perl 1.x and mod_perl 2.x on the same machine I would almost emphatically recommend using two different instances of perl since you will have some modules with the same name (Apache::Request for example) that are completely different versions (meaning A::R 1.x will not work with mod_perl 2.x and vice versa).