One way to approach this would be to ascertain what coverage you currently have and then keep writing tests until you have satisfactory coverage. Devel::Cover can help with this. For example, here are the first few lines of a table that lists the sort of coverage you would get:

File	stmt	branch	cond	sub	pod	time	total
/dp/usr/cxp/code/util_src/rtk_test/web_test	81.2	36.7	43.5	80.0	n/a	54.8	60.4
/home/cxp/code/th_web_tests/perl_lib/Aliased.pm	44.4	50.0	n/a	12.5	n/a	0.4	34.0
/home/cxp/code/th_web_tests/perl_lib/RTK/Class/MethodMaker.pm	73.7	72.7	n/a	55.6	n/a	1.5	71.6

I've left in the hyperlinks, but I changed them so that they don't actually go anywhere. Were you to actually generate such a report, those hyperlinks would go down into line-by-line detail pages that show you exactly what is and is not getting covered. You'll even find truth tables that show you which conditions of a logical expression are getting used!

By reading through Devel::Cover::Tutorial, you can learn what those headings mean. Generating such a report is easy. Here's how I did it:

perl -MDevel::Cover /usr/local/bin/web_test -d -h

# wait a while for the tests to finish

cover cover_db -report html
[download]

I number at the far right is a loose estimate of the percentage of code coverage. It's also fair to note that there are a few bugs in this code (it's alpha code, after all). That report came up fine the first time that I ran it. The second time, I found that the index file had been named .html, making it invisible with a standard ls. I had to rename it to index.html and copy it to the cover_db/ directory to get it to work properly.

Despite a few bugs, it's very easy to work with and can give you a nice idea of how complete your coverage is. Since the html is generated through Template Toolkit, you might even be able to auto-generate a few tests with this ...