The main difference between system() and exec() is that exec() forks the child and does not wait for a return, meaning it does not wait until the child process does it's thing and then continue. The system() command does just that, When you use system() the call waits for the child process to do it's thing and then reports the exit status of the child. If the child fails it reports whatever failed status the child exited with.

Hell it could even be a status of zero. You could explicitly assign an exit(0) to an error exit if you so choose. Take the following code fore example:

#!/usr/bin/perl -w

use strict;
my $var = 1;

if ($var == 1) {
   print "Hello World!\n";
}else{
   exit(0);
}
[download]

In this scenario if the program fails to do what it is suppose to do it will exit with a status of 0. Now this script did not fail to run but if you changed the value of $var it would failed to do what it was suppose to do.

exec() does not return the exit status of the child because , as already explianed, it does not wait for this status report. Instead it returns the status of the forking process itself. If it was able to fork the process into the background it returns a successfull status, if it does not it returns an error status.