I agree with the others that the simple chmod -R 775 from the command line would be easiest. But, if for some reason you need to use File::Find, I believe (I didn't refer back to the docs, or test this) the syntax is something to the effect of

find({chmod(0775, $_)}, ['/path/to/your/base/dir']);

I haven't used File::Find for a looooong time so I could be off with that. starting point though? :) YMMV