Straight out of system doc (my own emphasis added):
in reply to Capture output from command under su
The return value is the exit status of the program as returned by the wait call. To get the actual exit value, shift right by eight (see below). See also exec. This is not what you want to use to capture the output from a command, for that you should use merely backticks or qx//, as described in "`STRING`" in perlop. Return value of -1 indicates a failure to start the program or an error of the wait(2) system call (inspect $! for the reason).