perldelta entry for the PV/IV flags changes.

This needs an update to known_pod_issues.dat because the Pod has trailing
whitespace to avoid parts of the same verbatim code block getting separated.

Author: nwc10

pod/perldelta.pod

@@ -348,7 +348,119 @@ well.
 
 =item *
 
-XXX
+Reading the string form of an integer value no longer sets the flag C<SVf_POK>.
+The string form is still cached internally, and still re-read directly by the
+macros C<SvPV(sv)> I<etc> (inline, without calling a C function). XS code that
+already the APIs to get values will not be affected by this change. XS code
+that accesses flags directly instead of using API calls to express its intent
+I<might> break, but such code likely is already buggy if passed some other
+values, such as floating point values or objects with string overloading.
+
+This small change permits code (such as JSON serealisers) to reliably determine
+between
+
+=over 4
+
+=item *
+
+a value that was initially B<written> as an integer, but then B<read> as a string
+
+    my $answer = 42;
+    print "The answer is $answer\n";
+
+=item *
+
+that same value that was initially B<written> as a string, but then B<read> as an integer
+
+    my $answer = "42";
+    print "That doesn't look right\n"
+        unless $answer == 6 * 9;
+
+=back
+
+For the first case (originally written as an integer), we now have:
+
+    use Devel::Peek;
+    my $answer = 42;
+    Dump ($answer);
+    my $void = "$answer";
+    print STDERR "\n";
+    Dump($answer)
+    
+    
+    SV = IV(0x562538925778) at 0x562538925788
+      REFCNT = 1
+      FLAGS = (IOK,pIOK)
+      IV = 42
+    
+    SV = PVIV(0x5625389263c0) at 0x562538925788
+      REFCNT = 1
+      FLAGS = (IOK,pIOK,pPOK)
+      IV = 42
+      PV = 0x562538919b50 "42"\0
+      CUR = 2
+      LEN = 10
+
+For the second (originally written as a string), we now have:
+
+    use Devel::Peek;
+    my $answer = "42";
+    Dump ($answer);
+    my $void = $answer == 6 * 9;
+    print STDERR "\n";
+    Dump($answer)'
+    
+    
+    SV = PV(0x5586ffe9bfb0) at 0x5586ffec0788
+      REFCNT = 1
+      FLAGS = (POK,IsCOW,pPOK)
+      PV = 0x5586ffee7fd0 "42"\0
+      CUR = 2
+      LEN = 10
+      COW_REFCNT = 1
+    
+    SV = PVIV(0x5586ffec13c0) at 0x5586ffec0788
+      REFCNT = 1
+      FLAGS = (IOK,POK,IsCOW,pIOK,pPOK)
+      IV = 42
+      PV = 0x5586ffee7fd0 "42"\0
+      CUR = 2
+      LEN = 10
+      COW_REFCNT = 1
+
+(One can't rely on the presence or absence of the flag C<SVf_IsCOW> to
+determine the history of operations on a scalar.)
+
+Previously both cases would be indistinguishable, with all 4 flags set:
+
+    SV = PVIV(0x55d4d62edaf0) at 0x55d4d62f0930
+      REFCNT = 1
+      FLAGS = (IOK,POK,pIOK,pPOK)
+      IV = 42
+      PV = 0x55d4d62e1740 "42"\0
+      CUR = 2
+      LEN = 10
+
+(and possibly C<SVf_IsCOW>, but not always)
+
+This now means that if XS code I<really> needs to determine which form a value
+was first written as, it should implement logic roughly
+
+    if (flags & SVf_IOK|SVf_NOK) && !(flags & SVf_POK)
+        serealise as number
+    else if (flags & SVf_POK)
+        serealise as string
+    else
+        the existing guesswork ...
+
+Note that this doesn't cover "dualvars" - scalars that report different
+values when asked for their string form or number form (such as C<$!>).
+Most serialisation formats cannot represent such duplicity.
+
+I<the existing guesswork> remains because as well as dualvars, values might
+be C<undef>, references, overloaded references, typeglobs and other things that
+Perl itself can represent but do not map one-to-one into external formats, so
+need some amount of approximation or encapsulation.
 
 =back
 

t/porting/known_pod_issues.dat

@@ -388,6 +388,7 @@ pod/perlandroid.pod	Verbatim line length including indents exceeds 78 by	3
 pod/perlbook.pod	Verbatim line length including indents exceeds 78 by	1
 pod/perldebguts.pod	Verbatim line length including indents exceeds 78 by	24
 pod/perldebtut.pod	Verbatim line length including indents exceeds 78 by	2
+pod/perldelta.pod	line containing nothing but whitespace in paragraph	6
 pod/perldtrace.pod	Verbatim line length including indents exceeds 78 by	7
 pod/perlgit.pod	? Should you be using F<...> or maybe L<...> instead of	3
 pod/perlgit.pod	Verbatim line length including indents exceeds 78 by	1