What does a dollar sign followed by a question mark mean in Perl?

Question

In the following in Perl script:

$a=apple
$b=orange

if ($?==0) {
  # do something
}

What does $? mean here?

Answer 1

$? or $CHILD_ERROR (if use English) contains the status of the last backtic (or several means of running a child process from Perl.) See perlvar for a full explanation.