ls(1) runs in a child process, it is an external program. This is the case whether you run it from bash or perl. cd is a shell built-in, and does not create a child process. It alters the current directory, which is then inherited by any child processes. I am not sure what you intend to do - running built-ins in the same shell is fine, just redirect the shell's STDIN using open with a pipe, then print the commands to the pipe handle. Not sure what you will achieve running external programs this way though (to be honest I'm not sure why you want to use a shell at all).

Maybe this is semantics. A subshell is a shell mechanism where a separate environment is created, but a child process is not fork'ed if all the commands are built-ins. Think of it as an evnironment stack - changes are lost when the stack is collapsed at the end of the subshell.