If it really conforms to POSIX exit codes, then you need much more complex handling than just inverting with "not", e.g.

CheckExitCode( $item -> frobinize(...))
  or die($libobj->GetErrorCode);

sub CheckExitCode {
  if ($_[0] == -1) {
    print STDERR "POSIX: failed to execute\n";
        return 0;
  }
  elsif ($_[0] & 127) {
    printf STDERR "POSIX: child died with signal %d, %s coredump\n",
        ($_[0] & 127),
            ($_[0] & 128) ? 'with' : 'without';
        return 0;
  }
  elsif ( !$_[0] ) {
    printf STDERR "POSIX: child exited with value %d\n", $_[0] >> 8;
        return 0;
  }
  1;
}
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