The only direct-from-the-OP way I can think of to avoid copying is to create a set of manipulation routines in C, which is capable of directly addressing the shared memory, and call those from Perl to do anything with the data.

One other idea, depending how picky you are about your definition of "shared memory" ... instead of using explicit shared memory, use data segments in a shared library. This will create access to the functions in the library and function names will get copied (e.g. using P5NCI::Library), but provided you use the right C-compiler directives when declaring the data, the operating system (*) will only load one copy of those segments into memory, irrespective of concurrent users.

(update * I say "the operating system" but I take it from various clues that it's Unix of course ;))