If your data has no use outside the module it's tied to, place it somewhere nearby under $PERL5LIB. If the data has a life beyond interacting with your module (or if your module has a life beyond acting on this one data file), I would recommend keeping it out of your library hierarchy all together, and feeding it's location to your module (rather than hard coding it).

For my own projects I tend to do this:

Projects/
    MyProject/
        lib/   <-- project specific modules
        bin/   <-- project scripts
        data/  <-- project datafiles