because you cannot "decode()" a string into perl-internal utf8 if it is already flagged as being perl-internal utf8.

Not true. It would be a bug if the internal representation mattered (by definition of internal), and the following demonstrates that it doesn't. 

use strict;
use warnings;

use Encode qw( encode decode );

sub test {
   my ($enc, $orig) = @_;

   my $bin = encode($enc, $orig);

   utf8::downgrade my $bin_dn = $bin;  # UTF8=0
   utf8::upgrade   my $bin_up = $bin;  # UTF8=1

   my $txt_dn = decode($enc, $bin_dn);
   my $txt_up = decode($enc, $bin_up);

   printf("%-11s %d %d\n",
      "$enc:",
      $txt_dn eq $orig ? 1 : 0,
      $txt_up eq $orig ? 1 : 0,
   );
}

test('iso-8859-1', "A\x{E2}");
test('UTF-8',      "A\x{E2}\x{2660}");
test('UTF-16le',   "A\x{E2}\x{2660}");

[download]
iso-8859-1: 1 1
UTF-8:      1 1
UTF-16le:   1 1

[download]

I think you were trying to say that it makes no sense to decode something that's already been decoded, but that's got nothing to do with whether it's a "perl-internal utf8" buffer or not.

In reply to Re^2: question about Encode::decode('iso-8859-1', ...) by ikegami
in thread question about Encode::decode('iso-8859-1', ...) by perl5ever

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