#!/usr/bin/perl
use strict;
use warnings;
use CGI;

my $q = CGI->new();
our $cookie = '';

if ( defined $q->cookie('cookiename') ) {
    $cookie = $q->cookie('cookiename')
    # validate cookie here based on whatever
}
elsif ($q->param('some_field_name') {
    # form submitted

    $cookie = 'whatever';
}
else {
    # show the html form
}