---------                  -----------------
          |         |   form data    |                  |
          |         | -------------> | stdin            |
          |         |                |                  |
   -----> |         |   HTML content |           CGI    |
  browser |  Apache | <------------- | stdout    script |
   <----- |         |                |                  |
          |         |   error msgs   |                  |
          |         | <------------- | stderr           |
          |         |                |                  |
           ---------                  -----------------

##</code><code>##

           ---------                  -----------------
          |         |   form data    |                  |
          |         | -------------> |    -----------------
          |         | -----------------> |                 |
   -----> |         |   HTML content |   |                 |
  browser |  Apache | <------------- |   |                 |     
   <----- |         | <----------------- |   child         |
          |         |   error msgs   |   |   process       |
          |         | <------------- |   |                 |
          |         | <----------------- |                 |
           ---------                  ---|                 |
                                          -----------------

##</code><code>##

my $pid = fork;
die "fork failed: $!" unless defined($pid);
if ( $pid == 0 ) {
    # child
    close STDOUT;    # <--- !!
    close STDERR;    # <--- !!
    ...
    exec $program, @args;
    die "exec failed";
}