In the olden days, to make a patch to a module, you had to have the original, untouched file and a copy that you modified. You'd then use
diff to compare the two files.
At the Pittsburgh Perl Workshop, Ricardo was asking how to do some odd thing in git. Instead of anyone answering his question, everyone asked what he was doing. It turns out he was patching someone's module and making it a git repo while he worked. The process is really handy:
git commit -m "* Version 1.23 from CPAN"
git format-patch --stdout -1
There are other ways that you can do this, and you can change around the process in git. I like that git is lightweight enough to make it actually useful for everyday work.