I highly recommend the Perl Testing: A Developer's Notebook book. It's a gold-mine for testing.
One of the things the book recommends using is Devel::Cover for code coverage metrics. It's a great solution. You can't use its reports as a boolean "are we done with writing unit tests" check, but it can give you a great global idea of where your application is weak on unit tests.
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code('Perl') || die;