Sometimes it is necessary, to install perl modules locally and not in the perl directory. E.g., this could be necessary if you don't have root access to a machine and the system administrator does not want to install what you need---hence you install it in your home directory. Perl organises the search path in a variable @INC and upon initialisation the directories in the environment variable PERL5LIB are included. So, install locally (using --prefix as usual) and set PERL5LIB appropriately and you're fine.
Other environment variables worth keeping in mind include PATH, MANPATH and LD_LIBRARY_PATH.