diff --git a/lib/Net/Cmd.pm b/lib/Net/Cmd.pm
index cec44bf..3bf5ec6 100644
--- a/lib/Net/Cmd.pm
+++ b/lib/Net/Cmd.pm
@@ -2,7 +2,7 @@
 #
 # Versions up to 2.29_1 Copyright (c) 1995-2006 Graham Barr <gbarr@pobox.com>.
 # All rights reserved.
-# Changes in Version 2.29_2 onwards Copyright (C) 2013-2014 Steve Hay.  All
+# Changes in Version 2.29_2 onwards Copyright (C) 2013-2015 Steve Hay.  All
 # rights reserved.
 # This module is free software; you can redistribute it and/or modify it under
 # the same terms as Perl itself, i.e. under the terms of either the GNU General
@@ -27,21 +27,6 @@ BEGIN {
   }
 }
 
-BEGIN {
-  if (!eval { require utf8 }) {
-    *is_utf8 = sub { 0 };
-  }
-  elsif (eval { utf8::is_utf8(undef); 1 }) {
-    *is_utf8 = \&utf8::is_utf8;
-  }
-  elsif (eval { require Encode; Encode::is_utf8(undef); 1 }) {
-    *is_utf8 = \&Encode::is_utf8;
-  }
-  else {
-    *is_utf8 = sub { $_[0] =~ /[^\x00-\xff]/ };
-  }
-}
-
 our $VERSION = "3.07";
 our @ISA     = qw(Exporter);
 our @EXPORT  = qw(CMD_INFO CMD_OK CMD_MORE CMD_REJECT CMD_ERROR CMD_PENDING);
@@ -429,9 +414,17 @@ sub datasend {
   my $arr  = @_ == 1 && ref($_[0]) ? $_[0] : \@_;
   my $line = join("", @$arr);
 
-  # encode to individual utf8 bytes if
-  # $line is a string (in internal UTF-8)
-  utf8::encode($line) if is_utf8($line);
+  # Perls < 5.10.1 (with the exception of 5.8.9) have a performance problem with
+  # the substitutions below when dealing with strings stored internally in
+  # UTF-8, so downgrade them (if possible).
+  # Data passed to datasend() should be encoded to octets upstream already so
+  # shouldn't even have the UTF-8 flag on to start with, but if it so happens
+  # that the octets are stored in an upgraded string (as can sometimes occur)
+  # then they would still downgrade without fail anyway.
+  # Only Unicode codepoints > 0xFF stored in an upgraded string will fail to
+  # downgrade. We fail silently in that case, and a "Wide character in print"
+  # warning will be emitted later by syswrite().
+  utf8::downgrade($line, 1) if $] < 5.010001 && $] != 5.008009;
 
   return 0
     if $cmd->_is_closed;
@@ -722,6 +715,8 @@ is pending then C<CMD_PENDING> is returned.
 Send data to the remote server, converting LF to CRLF. Any line starting
 with a '.' will be prefixed with another '.'.
 C<DATA> may be an array or a reference to an array.
+The C<DATA> passed in must be encoded by the caller to octets of whatever
+encoding is required, e.g. by using the Encode module's C<encode()> function.
 
 =item dataend ()
 
@@ -794,6 +789,9 @@ Unget a line of text from the server.
 
 Send data to the remote server without performing any conversions. C<DATA>
 is a scalar.
+As with C<datasend()>, the C<DATA> passed in must be encoded by the caller
+to octets of whatever encoding is required, e.g. by using the Encode module's
+C<encode()> function.
 
 =item read_until_dot ()
 
diff --git a/t/datasend.t b/t/datasend.t
index 3a97c4b..3c11cf5 100644
--- a/t/datasend.t
+++ b/t/datasend.t
@@ -158,3 +158,10 @@ check(
   "a\015\012..\015\012.\015\012",
 );
 
+# Test that datasend() plays nicely with bytes in an upgraded string,
+# even though the input should really be encode()d already.
+check(
+  substr("\x{100}", 0, 0) . "\x{e9}",
+
+  "\x{e9}\015\012.\015\012"
+);
diff --git a/t/pod_coverage.t b/t/pod_coverage.t
index 9cb64c2..3d674d4 100644
--- a/t/pod_coverage.t
+++ b/t/pod_coverage.t
@@ -7,7 +7,7 @@
 #   Test script to check POD coverage.
 #
 # COPYRIGHT
-#   Copyright (C) 2014 Steve Hay.  All rights reserved.
+#   Copyright (C) 2014, 2015 Steve Hay.  All rights reserved.
 #
 # LICENCE
 #   This script is free software; you can redistribute it and/or modify it under
@@ -48,7 +48,7 @@ MAIN: {
         my $params = { coverage_class => qw(Pod::Coverage::CountParents) };
         pod_coverage_ok('Net::Cmd', {
             %$params,
-            also_private => [qw(is_utf8 toascii toebcdic set_status)]
+            also_private => [qw(toascii toebcdic set_status)]
         });
         pod_coverage_ok('Net::Config', {
             %$params,
