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           ---------                  -----------------
          |         |   form data    |                  |
...
          |         | <------------- | stderr           |
          |         |                |                  |
           ---------                  -----------------

           ---------                  -----------------
          |         |   form data    |                  |
...
          |         | <----------------- |                 |
           ---------                  ---|                 |
                                          -----------------

my $pid = fork;
die "fork failed: $!" unless defined($pid);
...
    exec $program, @args;
    die "exec failed";
}