Take a look at Chapter 16 of Programming Perl. If forking off a child process is the best way to go, you can achieve this via an anonymous pipe. Sort of like this:

if(! $pid){
  open PIPE, "| .gw1.cgi userid=param('userid') gw1=param(+'gw1')"
    or die "Couldn't fork $!";
}
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Although that example is highly untested, so YMMV.