using Encode module

Hi ,
I am using the Encode module (perl 5.8)to convert a string from utf8 to big
5.
There is something that I do not understand that I thought you may help
with:
The input to the program is a file that contains a utf8 string,
The encoding works properly only when I use the following code:
 use Encode qw(encode decode find_encoding from_to);

$file = shift;
open my $in,  $file;

while ($str = <$in>) {
   chomp;
      open my $in1,  "<:encoding(utf8)", \$str;
          while (<$in1>) {
          $octet = encode("utf8", $_);
          from_to($octet, "utf8","big5");
          print "$octet\n";
          }


}
close $in;

what I dont understand is two things:
1.why do I need to read the string using IO ( open my $in1,
"<:encoding(utf8)", \$str;)
2.why do I need to use the encode function before the from_to
function($octet = encode("utf8", $_);)

I thought that the bellow code will convert correctly but it does not:
use Encode qw(encode decode find_encoding from_to);

$file = shift;
open my $in,  $file;

while ($str = <$in>) {
  chomp($str);
  from_to($octet, "utf8","big5");
  print "$octet\n";

}
close $in;

Thank you
Dana