As per this page : http://perldoc.perl.org/functions/fork.html, fork() "returns the child pid to the parent process, 0 to the child process, or undef if the fork is unsuccessful", so you can tell which 'copy' you are currently in, as it were ...
Sort of like this

        # Fork new process
        $pid = '';
        if( $pid = fork() )
        {
            # Parent proc

        }
        elsif( !defined $pid )
        {
            exit_with_error("Can't fork: $!");
        }

        # ... else we are the child, call exec.
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Cheers
Chris