use CGI;
use CGI::Carp qw / fatalsToBrowser /; # not in production!

my $cgi= new CGI;

#
# stuff

if ( not $cgi->param('paramIReallyNeed')){
   logexeception('paramIReallyNeed was missing from page');
    displayExeptionPage($cgi);
    # Won't return... displayExceptionPage exits.
}

# more code follows.