use CGI ':cgi';

# list of parameter names
my @params = qw(city zip lastname country other obs);

# extract all parameters into a hash
my %param;
foreach (@params) {
  $param{$_} = param($_);
}

# time passes...

while (<MYTEMPLATE>) {
  foreach my $p (@params) {
    s/<\?--$p-->/$param{$p}/g if defined $param{$p};
    # you'll probably want to print it too
  }
}