[perl #119123] disallow literal control character variables

This introduces a deprecation warning on things like $^T, where ^T
is a literal control character in the source code.

pod/perldata.pod

@@ -207,6 +207,9 @@ property, like C<$!> or C<%+>.
 
 =back
 
+Note that as of Perl 5.20, literal control characters in variable names
+are deprecated.
+
 =head2 Context
 X<context> X<scalar context> X<list context>
 

pod/perldelta.pod

@@ -139,7 +139,18 @@ numeric values under the hood.)
 
 =head1 Deprecations
 
-XXX Any deprecated features, syntax, modules etc. should be listed here.
+=head2 Literal control characters in variable names
+
+This deprecation affects things like C<$\cT>, where \cT is a literal
+control in the source code.  Surprisingly, it appears that originally
+this was intended as the canonical way of accessing variables like
+C<$^T>, with the caret form only being added as an alternative.
+
+The literal control form is being deprecated for two main reasons: It
+has what are likely unfixable bugs, such as C<$\cI> not working as an
+alias for C<$^I>, and their usage not being portable to non-ASCII
+platforms: While C<$^T> will work everywhere, C<\cT> is whitespace in
+EBCDIC.
 
 =head2 Module removals
 

pod/perldiag.pod

@@ -6077,6 +6077,13 @@ startup.
 In code that currently says C<use AutoLoader; @ISA = qw(AutoLoader);>
 you should remove AutoLoader from @ISA and change C<use AutoLoader;> to
 C<use AutoLoader 'AUTOLOAD';>.
+
+=item Use of literal control characters in variable names is deprecated
+
+(D deprecated) Using literal control characters in the source to refer
+to the ^FOO variables, like C<$^X> and C<${^GLOBAL_PHASE}> is now
+deprecated.  This only affects code like C<$\cT>, where \cT is a control in
+the source code: C<${"\cT"}> and C<$^T> remain valid.
 
 =item Use of %s in printf format not supported
 

t/base/lex.t

@@ -1,6 +1,6 @@
 #!./perl
 
-print "1..94\n";
+print "1..91\n";
 
 $x = 'x';
 
@@ -129,16 +129,6 @@ my $test = 31;
   if ($ {^XY} != 23) { print "not "  }
   print "ok $test\n"; $test++;
 
-# Does the syntax where we use the literal control character still work?
-  if (eval "\$ {\cX}" != 17 or $@) { print "not "  }
-  print "ok $test\n"; $test++;
-
-  eval "\$\cQ = 24";                 # Literal control character
-  if ($@ or ${"\cQ"} != 24) {  print "not "  }
-  print "ok $test\n"; $test++;
-  if ($^Q != 24) {  print "not "  }  # Control character escape sequence
-  print "ok $test\n"; $test++;
-
 # Does the old UNBRACED syntax still do what it used to?
   if ("$^XY" ne "17Y") { print "not " }
   print "ok $test\n"; $test++;

t/lib/warnings/toke

@@ -148,6 +148,32 @@ EXPECT
 Use of bare << to mean <<"" is deprecated at - line 2.
 ########
 # toke.c
+eval "\$\cT";
+eval "\${\7LOBAL_PHASE}";
+eval "\${\cT}";
+eval "\${\n\cT}";
+eval "\${\cT\n}";
+my $ret = eval "\${\n\cT\n}";
+print "ok\n" if $ret == $^T;
+
+no warnings 'deprecated' ;
+eval "\$\cT";
+eval "\${\7LOBAL_PHASE}";
+eval "\${\cT}";
+eval "\${\n\cT}";
+eval "\${\cT\n}";
+eval "\${\n\cT\n}";
+
+EXPECT
+Use of literal control characters in variable names is deprecated at (eval 1) line 1.
+Use of literal control characters in variable names is deprecated at (eval 2) line 1.
+Use of literal control characters in variable names is deprecated at (eval 3) line 1.
+Use of literal control characters in variable names is deprecated at (eval 4) line 1.
+Use of literal control characters in variable names is deprecated at (eval 5) line 1.
+Use of literal control characters in variable names is deprecated at (eval 6) line 1.
+ok
+########
+# toke.c
 $a =~ m/$foo/eq;
 $a =~ s/$foo/fool/seq;
 
@@ -1379,3 +1405,14 @@ q
 1
 1
 q
+########
+# toke.c
+#[perl #119123] disallow literal control character variables
+eval "\$\cQ = 25";
+eval "\${ \cX } = 24";
+*{
+    Foo
+}; # shouldn't warn on {\n, even though \n is a control character
+EXPECT
+Use of literal control characters in variable names is deprecated at (eval 1) line 1.
+Use of literal control characters in variable names is deprecated at (eval 2) line 1.

t/op/svleak.t

@@ -37,6 +37,7 @@ sub leak {
 # if the name is absent.
 sub eleak {
     my ($n,$delta,$code,@rest) = @_;
+    no warnings 'deprecated'; # Silence the literal control character warning
     leak $n, $delta, sub { eval $code },
          @rest ? @rest : $code
 }

t/uni/variables.t

@@ -12,7 +12,7 @@ use utf8;
 use open qw( :utf8 :std );
 no warnings qw(misc reserved);
 
-plan (tests => 65873);
+plan (tests => 65878);
 
 # ${single:colon} should not be valid syntax
 {
@@ -217,6 +217,9 @@ EOP
 
     {
         no strict;
+        # Silence the deprecation warning for literal controls
+        no warnings 'deprecated';
+
         for my $var ( '$', "\7LOBAL_PHASE", "^GLOBAL_PHASE", "^V" ) {
             eval "\${ $var}";
             is($@, '', "\${ $var} works" );
@@ -242,7 +245,31 @@ EOP
         "...but \$^J is still legal"
     );
 
+    no warnings 'deprecated';
     my $ret = eval "\${\cT\n}";
     is($@, "", 'No errors from using ${\n\cT\n}');
     is($ret, $^T, "...and we got the right value");
 }
+
+{
+    # Originally from t/base/lex.t, moved here since we can't
+    # turn deprecation warnings off in that file.
+    no strict;
+    no warnings 'deprecated';
+
+    my $CX  = "\cX";
+    $ {$CX} = 17;
+
+    # Does the syntax where we use the literal control character still work?
+    is(
+       eval "\$ {\cX}",
+       17,
+       "Literal control character variables work"
+    );
+
+    eval "\$\cQ = 24";                 # Literal control character
+    is($@, "", "...and they can be assigned to without error");
+    is(${"\cQ"}, 24, "...and the assignment works");
+    is($^Q, 24, "...even if we access the variable through the caret name");
+    is(\${"\cQ"}, \$^Q, '\${\cQ} == \$^Q');
+}

toke.c

@@ -9437,6 +9437,10 @@ S_scan_ident(pTHX_ char *s, const char *send, char *dest, STRLEN destlen, I32 ck
     if (s < send
         && (isIDFIRST_lazy_if(s, is_utf8) || VALID_LEN_ONE_IDENT(*s, is_utf8)))
     {
+        if ( isCNTRL_A((U8)*s) ) {
+            deprecate("literal control characters in variable names");
+        }
+
         if (is_utf8) {
             const STRLEN skip = UTF8SKIP(s);
             STRLEN i;