I don't know much, if anything, about forking processes or creating threads in perl.

However, if each child process, i, is creating Y threads, then should the threads perhaps be created in the child process?

Then you'd have X(i)Y(j) for the thread id - X(i) being the id of the child and j being the id of the j'th thread created by child i, i=1..X and j=1..Y.

Think of it as a matrix or table with i,j identifying the matrix element or table cell running.

It's not clear to me if you need just all threads of child X(i) to be able to communicate, but they don't need to communicate with the threads of other children X(k),k=0..X, k!=i, i.e.

X(i)Y(j) can communicate with X(i)Y(k), k!=j but not X(k)Y(*), k!=i.

Or if you need all threads to be able to communicate to all other threads created by other children, i.e.

X(i)Y(j) can communicate with X(u)Y(v) and i,u=1..X, j,v=1..Y although i!=u and j!=v might still apply.