That's not right.

• Both qx and system set $! when unable to launch the child.
• Both qx and system set $? to the child's error code.
• An error did not necessarily occur when qx return false, only when it returns undef.
• $! is not necessarily set when qx return false, only when it returns undef.

Here is how one actually gets meaningful errors from them:

my $output = qx(date);
die("Unable to launch external process: $!\n")
   if not defined $output;
die("The external process did not run successfully (error code $?)\n")
   if $?;
[download]

my $rv = system(date);
die("Unable to launch external process: $!\n")
   if $rv == -1;
die("The external process did not run successfully (error code $?)\n")
   if $?;
[download]