use strict;
use warnings;
use CGI qw(:standard);

print header,start_html,start_form,
   p(
      'How many columns? ',
      popup_menu(-name=>'cols',-values=>[1..16]),
   ),
   submit,end_form,hr,
;

my $cols = param('cols') || 4;

my @one_d = map sprintf('%02x',$_) x 3, reverse (0..255);
my @two_d =
   map[ @one_d[$_..$_+$cols-1] ],
   range(0,$#one_d,$cols)
;

print p("there are $cols columns:"),
   table(
      map {Tr( map {td{bgcolor=>"#$_"},$_} @$_ )} @two_d
   ),
   end_html,
;

sub range {grep!(($_-$_[0])%($_[2]||1)),$_[0]..$_[1]}