how can i modify the @INC list to add "/usr/lib/perl5/5.8.3"

You can set the environment variable PERL5LIB to add directories to @INC, but it's probably almost certainly a better idea to install the Perl libs that belong to a certain Perl version right where the respective Perl binary looks anyway (those @INC paths you see, which are hardcoded in the binary — in your case /usr/local/lib/perl5/5.8.3/...).

(Update: and if you decide to use PERL5LIB, set it locally only for exactly the processes that need it (e.g. in wrapper scripts)...  Setting PERL5LIB globally on a system with multiple Perl versions is asking for trouble!)