If you're using ExtUtils::MakeMaker or Module::Build, then you can run all your tests automatically by entering the command "make test" or "Build test", which will execute any *.t files in your project's t/ subfolder.
If you're not using either of these, then you can use TAP::Harness to automate execution of multiple test scripts.
To actually write the tests, use Test::More or any of the modules that others have suggested here.
