What I've done to have separate Perl versions going at once, is to rename the executable. The executable knows where to find it's libs by the way it's compiled. So for instance if I was moving to perl58 from perl56, I would rename the original perl binary to perl56 and then install perl 5.8. When I want to use 5.6, I put #!/usr/bin/perl56 as the shebang line. When I want to use perl 5.8 I use the normal shebang, or I could use #!/usr/bin/perl58. You can even make a symlink @perl to point to which version you want to run as default.