In this particular situation, it looks like you just want to concatenate your cities and distances to those already chosen. You could use a single hidden field for each, something like:

my @cities,@distances;
if ($cgi->param('hiddenCities')) { # there are cities and distances
                                   # from before
    @cities = split(';',$cgi->param('hiddenCities'));
    @distances = split(';',$cgi->param('hiddenDistances'));
}
if ($cgi->param('Go')) { # form was submitted
    push @cities,$cgi->param('city');
    push @distances,$cgh->param('circularWarningDistanceStart');
}
# put list of cities back on the form
print $cgi->hidden('hiddenCities',join(';',@cities);
print $cgh->hidden('hiddenDistances',join(';',@distances);
# ....
# do more form stuff here
# like asking for more input, etc.
[download]

The code above is untested, but I hope that gives you some ideas you could use.

Sean