while (<INFILE>) {

    # remove leading whitespace & newline from end
    # and split the row on whitespace
    my @r = chomp && s/^\s+// && split /\s+/;
    
    # create a key in lut hash using rgb vals
    push @{$lut{"$r[1].$r[2].$r[3]"}}, $r[0];
}

while (my ($k, $v) = each %lut) {
    $k =~ s/\./ /g;     # replace . in hash key with space
    my @v = sort @$v;   # sort the color brightness array to get 
                        # min/max values
    
    print OUTFILE 
       "CLASS\n" . 
       "    EXPRESSION ([pixel] >= $v[0] AND [pixel] <= $v[$#v]\n" . 
       "    STYLE\n" . 
       "        COLOR $k\n" . 
       "    END\n" . 
       "END\n";
}