I will disagree with chromatic and btrott and say that I prefer to set the PERLLIB or PERL5LIB environment variable to the path where my modules are installed. Why? Because then you don't need to modify existing scripts to add the 'use lib' directive. The additional directories will be searched automatically.

Also, if you "use lib" and later you need to move your modules directory for some reason, you will have to go modify all your scripts. If you use the environment variable, you just have to change one place and all your scripts will keep happily running.