I'm not sure i understand what you mean when you say "group" but creating those rows of checkboxs shouldn't be too rough.
my @Days = ("Monday","Tuesday","Wednesday","Thursday","Friday","Saturd
foreach my $Day (@Days)
$html .= "<tr>\n\t<td>$Day</td>\n";
$html .= "\t<td><input type=checkbox name=$Day$_></td>\n";
$html .= "</tr>\n";
That generates code almost exactly like the current code. Of course you need to add the beginning of the form, and the table headers, a any formating options.
note: i added the \n and \t just for display purpouses they can obviously be removed.
You can remove the $_ from the naming of the boxes to have them be 'groups', i'm not sure how that effects how they are passed though, and you need to add values to distinguish between them :-)