There is a file called README.macosx. Did you READIT?

=head2 Installation Prefix

The default installation location for this release uses the traditiona
+l
UNIX directory layout under /usr/local. This is the recommended locati
+on
for most users, and will leave the Apple-supplied Perl and its modules
undisturbed.

Using an installation prefix of '/usr' will result in a directory layo
+ut
that mirrors that of Apple's default Perl, with core modules stored in
'/System/Library/Perl/${version}', CPAN modules stored in
'/Library/Perl/${version}', and the addition of
'/Network/Library/Perl/${version}' to @INC for modules that are stored
on a file server and used by many Macs.
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When you say not use any modules I assume you mean not use any NON CORE modules. The downside of 5.8.x is it is not binary compatible with 5.6 so you have to recompile all non core modules and reinstall them. Actually only those that have C source ie XS modules not Pure Perl. Thus if you achieved your goal of pointing to the old 5.6.x libraries you would be in for some nasty suprises (Pure Perl would be OK, compiled stuff could/should/would/might crash)

./Configure PREFIX=/usr
[download]

Given OS X is *nix and thus should support symlinks

ln -s /System/Library/Perl/5.8.2 /System/Library/Perl
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This aliases /System/Library/Perl to the real dir /System/Lobrary/Perl/5.8.2. but you probably don't wnat to do that. You typically symlink the perl binary into /usr/bin/perl or /usr/bin/local/perl for ease of use. Some people will link to /usr/bin/Perl5.8 so you can get either the original with /usr/bin/perl or 5.8 with ....