And, is that the only place where such a module could be found, or could it also be in a file of a different name and/or in a different subdirectory?

Well, this is just a technicality that only applies to require, not use, but just for completeness: You could take a module, for example Try::Tiny, copy its Tiny.pm to e.g. /tmp, and include it via BEGIN { require "/tmp/Tiny.pm"; Try::Tiny->import() } and it'll work fine - not that it's recommended :-)

If you only use use and modules from CPAN, then yes, the directory structure is pretty well defined (some rare, exotic modules may do things differently). The relevant documentation is use, require and @INC.