First of all, welcome to the wonderful world of testing! I recently started writing tests for my modules, and I'm hooked. :-)

References you might find helpful:

• Perl Testing: A Developer's Notebook, by Ian Langworth and chromatic (ISBN 0596100922)
• Kent Beck's Test-Driven Development (ISBN 0321146530)
• Andy Lester's presentation on automated testing

Relevant nodes found by Super Search (this is one of your best friends at Perl Monks):

• Testing: Fun for the family
• testing code
• Functional and Unit Test Redundancy
• What is the best way to add tests to existing code?
• Testing at the right granularity

Finally, don't forget to search CPAN:

• Devel::Cover
• Test::Tutorial
• Test::Simple
• Test::More
• ...and other members of the Test::* namespace

Your specific example of a function that checks bounds is certainly something that I would test. It will allow you to easily verify that it is behaving as expected (per the specifications), especially after you tweak the code (anything from a minor change to a complete refactoring).

For example: Does the function handle different types of input appropriately? What should happen if it is passed undef? An integer? A float? Does it work with positive and negative values? An alphanumeric string? A string that contains with special characters? The empty string? What other edge cases are there?

Just some initial thoughts. Hope this helps.