Here are some quick answers for you. For more indepth info just do a search of this site:

• system, stripped down of security features, is simply a fork() and exec(). fork() creates a new process space which exec() populates with whatever process you specified in the shell call.
• Backticks fire off the shell call in a separate process, then captures STDOUT from it.
• Threads have far less overhead than a process and are contained within the process which allows concurrent handling of instructions.
• You may fork() and specify further perl code instead of an exec so that you end up with multiple perl interpreters running in separate process spaces.
• At any time, you can call exec() which then "overwrites" the process space with a new binary with some exceptions like file descriptors. This is useful to redirect streams before handing over the streams to some process.

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