If you follow your "T=25,A=3,D=18" form, you can sort numerically easily enough. Let's say you wanted the data in order from lowest to highest temperature, and your data is in @data, with each element being a string of the above form:

my @sorted_by_temp_asc = sort { tricky_sort($a,$b,'T') } @data;

sub tricky_sort {
   my ($A, $B, $field) = @_;
   my %a_dat = map { split('=', $_, 2) } split(',',$A);
   my %b_dat = map { split('=', $_, 2) } split(',',$B);

   return ( $a_dat{$field} <=> $b_dat{$field} );
}
[download]

The tricky_sort subroutine peforms a numerical comparison on the identified field, thus handling all the +/- sorting the way you want, and without too much twisted logic. Of course, you'll probably want to add more data validation and such before you put this into production -- after all <grail>it's only a model</grail> ;-)

And, it probably goes without saying, but to do a descending sort, just swap the order in which you pass $a and $b to tricky_sort