There's no assumption being made as to which shell launched Perl. As documented, the shell used by system is constant.

On most platforms, that shell is /bin/sh (no matter what the user's login shell is or which process launched perl), so system's argument must be a sh command. & can be used to run a background task in sh.

On Windows (cygwin aside), that shell is cmd, so system's argument must be a cmd command. start can be used to run a background task in cmd.