Use PerlIO::utf8_strict as the default.

Leon suggested using PerlIO::utf8_strict as the default.
doherty#42 (comment)

It seems to work.

I'm assuming there's still value in trying to use :encoding(UTF-8)?
Maybe it's faster?

Makefile.PL

@@ -20,6 +20,7 @@ my %WriteMakefileArgs = (
     "Carp" => 0,
     "Encode" => 0,
     "Import::Into" => 0,
+    "PerlIO::utf8_strict" => 0,
     "Symbol" => 0,
     "charnames" => 0,
     "feature" => 0,
@@ -60,6 +61,7 @@ my %FallbackPrereqs = (
   "IPC::Open3" => 0,
   "Import::Into" => 0,
   "PerlIO" => 0,
+  "PerlIO::utf8_strict" => 0,
   "Symbol" => 0,
   "Test::Exception" => 0,
   "Test::Fatal" => 0,

lib/utf8/all.pm

@@ -167,7 +167,17 @@ sub import {
     my $utf8_IO_encoding = $class->_choose_utf8_IO_encoding;
 
     'utf8'->import::into($target);
-    'open'->import::into($target => $utf8_IO_encoding, ':std');
+    'open'->import::into($target => "IO" => $utf8_IO_encoding);
+
+    # use open ':std' only works with some encodings.
+    state $have_encoded_std = 0;
+    if( !$have_encoded_std ) {
+        binmode STDERR, $utf8_IO_encoding;
+        binmode STDOUT, $utf8_IO_encoding;
+        binmode STDIN,  $utf8_IO_encoding;
+        $have_encoded_std = 1;
+    }
+
     'charnames'->import::into($target, qw{:full :short});
     'warnings'->import::into($target, qw{FATAL utf8});
     'feature'->import::into($target, qw{unicode_strings}) if $^V >= v5.11.0;
@@ -272,7 +282,8 @@ sub _choose_utf8_IO_encoding {
     return ':encoding(UTF-8)' if $^V >= v5.24.0 || (!$Config{usethreads} && !$Config{useithreads});
 
     # A safe default.
-    return ':utf8';
+    require PerlIO::utf8_strict;
+    return ':utf8_strict';
 }
 
 =head1 INTERACTION WITH AUTODIE

t/lexical-again.t

@@ -30,7 +30,7 @@ is "テスト" => $expected_unicode, 'Literal string should be characters under
         my @layers = PerlIO::get_layers($handles{$fh});
         ok(!grep(m/utf8/, @layers), "$fh: utf8 does not appear in the perlio layers")
             or diag explain { $fh => \@layers };
-        ok(!grep(m/utf-8-strict/, @layers), "$fh: utf-8-strict does not appear in the perlio layers")
+        ok(!grep(m/utf-?8[-_]strict/, @layers), "$fh: utf-?8[-_]strict does not appear in the perlio layers")
             or diag explain { $fh => \@layers };
     }
 
@@ -52,6 +52,8 @@ for my $fh (keys %handles) {
     my @layers = PerlIO::get_layers($handles{$fh});
     ok(grep(m/utf8/, @layers), "$fh: utf8 does appear in the perlio layers")
         or diag explain { $fh => \@layers };
+    ok(grep(m/utf-?8[-_]strict/, @layers), "$fh: utf-?8[-_]strict does appear in the perlio layers")
+        or diag explain { $fh => \@layers };
 }
 
 done_testing;

t/open.t

@@ -8,6 +8,8 @@ ok open my $in, '<', 'corpus/testfile';
 my @layers = PerlIO::get_layers($in);
 ok(grep(m/utf8/, @layers), 'utf8 appears in the perlio layers')
     or diag explain { $fh => \@layers };
+ok(grep(m/utf-?8[-_]strict/, @layers), 'utf-?8[-_]strict appears in the perlio layers')
+    or diag explain { $fh => \@layers };
 
 my $contents = do { local $/; <$in>};
 is $contents, "\x{30c6}\x{30b9}\x{30c8}\n", 'unicode retrieved OK';

t/utf8.t

@@ -19,6 +19,8 @@ use Test::More;
         my @layers = PerlIO::get_layers($fh);
         ok(grep(m/utf8/, @layers), 'utf8 appears in the perlio layers')
             or diag explain { $fh => \@layers };
+        ok(grep(m/utf-?8[-_]strict/, @layers), 'utf-?8[-_]strict appears in the perlio layers')
+            or diag explain { $fh => \@layers };
     }
 }
 
@@ -31,14 +33,14 @@ use Test::More;
     END { unlink "perlio_test2" }
 
     my @layers = PerlIO::get_layers($test_fh);
-    SKIP: {
+  SKIP: {
         # If we have the Perl Unicode flag set that adds the UTF-8 layer,
         # we need to skip this test.
         skip 'Perl Unicode flag set that always adds UTF-8 layer to output', 1 if (${^UNICODE} & 16);
         ok( !grep(/utf8/, @layers), q{utf8 doesn't appear in perlio layers})
             or diag explain { $test_fh => \@layers };
     }
-    ok( !grep(m/utf-8-strict/, @layers), q{utf-8-strict doesn't appear in the perlio layers})
+    ok( !grep(m/utf-?8[-_]strict/, @layers), q{utf-?8[-_]strict doesn't appear in the perlio layers})
         or diag explain { $test_fh => \@layers };
 
 }