Nice catch on the return value of system--but in error cases, it doesn't set $!, but rather $?. Unfortunately, $? doesn't stringify nearly as nicely as $!, but you can get reasonable information (as outlined in perlvar) with a little extra work:

unless (0 == system $cmd) {
   warn sprintf "External command '%s' failed with exit status %d (sig
+nal %d).  A core dump %s\n",
   $cmd, $? >> 8, $? & 127, $? & 128 ? "occurred" : "did not occur";
}
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