This leads me to the question: how does qx select the aforementioned "/bin/sh equivalent"?

The shell that is used is determined at the time that the perl executable was compiled. On Unix-like systems it will probably be /bin/sh, but on other platforms (OS2, Win32, VMS) it could be very different.

Also, the shell is not guaranteed to be "equivalent" to /bin/sh. On non-Unix systems perhaps "counterpart" is a better way to describe it.