The run-time library chops it up to populate argc/argv before calling main.

As for when Perl uses a shell vs. calls the process directly, there is lots of OS-specific case code in there. I looked through it for Windows to see what it really did, and probably posted an expose here in PM, but I can't remember the details. Every other OS has its own special stuff, too, so it's quite non-portable to make such assumptions.