Excellent explanation.

Another alternative is to write STDOUT and/or STDERR to a File::Temp.

Stricly speaking, the system buffer size for pipes enters into the pictures so you can squeak by if the program reads all but 512 bytes (for example) of its input before it writes more than 512 bytes of output. But this usually just leads one to think that they have a robust solution when they really don't rather than being truely helpful.