Use it when you are trying to see just what kind of gunk^Wdirectory paths %INC is filled up with, when you know you have installed a module but perl is failing to find one.

Actually, in such a case @INC is far more useful than %INC. %INC just gives you information where Perl actually has found a module it loaded - it doesn't give you much useful information if it couldn't find a module.

Other similar use is when trying to sort out the module paths, say to clean up the duplicates.

Again, in that case, @INC is far more useful than %INC.