my @keys  = sort keys %hash;
my $group = 0;

while (my @group = splice(@keys, 0, 20)) {

  ## generate url from stuff in @group
  $group++;

  ## maybe something like this if the items 
  ## are all safe (\w characters):
  my $url = 'script.cgi?' . join '&', map { "x=$_" } @group;

  print qq(<a href="$url">group # $group</a><br>\n);

}