Document that the -Ci, -Co, and -CD options have file scope

While the other -C settings are global, "i" and "o" only affect subsequent
open() calls in the current file. Document this and add two tests to
make sure the documented behaviour doesn't change accidentally.

pod/perlrun.pod

@@ -301,11 +301,11 @@ STDOUT and STDERR.  Repeating letters is just redundant, not cumulative
 nor toggling.
 
 The C<io> options mean that any subsequent open() (or similar I/O
-operations) will have the C<:utf8> PerlIO layer implicitly applied
-to them, in other words, UTF-8 is expected from any input stream,
-and UTF-8 is produced to any output stream.  This is just the default,
-with explicit layers in open() and with binmode() one can manipulate
-streams as usual.
+operations) in the current file scope will have the C<:utf8> PerlIO layer
+implicitly applied to them, in other words, UTF-8 is expected from any
+input stream, and UTF-8 is produced to any output stream.  This is just
+the default, with explicit layers in open() and with binmode() one can
+manipulate streams as usual.
 
 C<-C> on its own (not followed by any number or option list), or the
 empty string C<""> for the C<PERL_UNICODE> environment variable, has the

t/run/switchC.t

@@ -18,11 +18,12 @@ BEGIN {
 
 BEGIN { require "./test.pl"; }
 
-plan(tests => 11);
+plan(tests => 13);
 
 my $r;
 
 my $tmpfile = tempfile();
+my $scriptfile = tempfile();
 
 my $b = pack("C*", unpack("U0C*", pack("U",256)));
 
@@ -58,6 +59,20 @@ $r = runperl( switches => [ '-Ci', '-w' ],
               stderr   => 1 );
 like( $r, qr/^256(?:\r?\n)?$/s, '-Ci: auto-UTF-8 open for input' );
 
+open(S, ">$scriptfile") or die("open $scriptfile: $!");
+print S "open(F, q(<$tmpfile)); print ord(<F>); close F";
+close S;
+
+$r = runperl( switches => [ '-Ci', '-w' ],
+	      progfile => $scriptfile,
+              stderr   => 1 );
+like( $r, qr/^256(?:\r?\n)?$/s, '-Ci: auto-UTF-8 open for input affects the current file' );
+
+$r = runperl( switches => [ '-Ci', '-w' ],
+	      prog     => "do q($scriptfile)",
+              stderr   => 1 );
+unlike( $r, qr/^256(?:\r?\n)?$/s, '-Ci: auto-UTF-8 open for input has file scope' );
+
 $r = runperl( switches => [ '-CA', '-w' ],
 	      prog     => 'print ord shift',
               stderr   => 1,