If you can't justify the benefits of your approach towards your boss, turning to us is a bad technique.
Personally, the make, make install is just too slow for a rapid edit, run, edit cycle. It would make me 20% slower with no apparent gain. - why do you think you need to install just for testing? Just run your tests using perl -I lib from your distribution directory and you're all set. Your argument regarding the MANIFEST is a strawman - you can easily write a Perl program to maintain your MANIFEST file.