Store PL_prevailing_version in the frozen hinthash around a string-ev…

…al op

We need to save the value of PL_prevailing_version at the time the eval
op was compiled, so it can be put in place during the running code.

Ideally we'd do something more robust, like change the OP_ENTERVAL op
class into UNOP_AUX, so that the aux vector can store additional
information like the version number and perhaps the frozen hints hash.

In practice it is far too close to the 5.40 release to contemplate such
a change now, so this is a less intrusive but hackier change to achieve
the same aim.

See also
  #22121

op.c

@@ -12416,6 +12416,8 @@ Perl_ck_eval(pTHX_ OP *o)
     PERL_ARGS_ASSERT_CK_EVAL;
 
     PL_hints |= HINT_BLOCK_SCOPE;
+    if(PL_prevailing_version != 0)
+        PL_hints |= HINT_LOCALIZE_HH;
     if (o->op_flags & OPf_KIDS) {
         SVOP * const kid = cSVOPx(cUNOPo->op_first);
         assert(kid);
@@ -12457,6 +12459,7 @@ Perl_ck_eval(pTHX_ OP *o)
      && !(o->op_private & OPpEVAL_COPHH) && GvHV(PL_hintgv)) {
         /* Store a copy of %^H that pp_entereval can pick up. */
         HV *hh = hv_copy_hints_hv(GvHV(PL_hintgv));
+        hv_stores(hh, "CORE/prevailing_version", newSVuv(PL_prevailing_version));
         OP *hhop;
         hhop = newSVOP(OP_HINTSEVAL, 0, MUTABLE_SV(hh));
         /* append hhop to only child  */

pp_ctl.c

@@ -4062,6 +4062,7 @@ S_doeval_compile(pTHX_ U8 gimme, CV* outside, U32 seq, HV *hh)
     else {
         PL_hints = saveop->op_private & OPpEVAL_COPHH
                      ? oldcurcop->cop_hints : (U32)saveop->op_targ;
+        PL_prevailing_version = 0; /* we might change this below */
 
         /* making 'use re eval' not be in scope when compiling the
          * qr/mabye_has_runtime_code_block/ ensures that we don't get
@@ -4076,6 +4077,11 @@ S_doeval_compile(pTHX_ U8 gimme, CV* outside, U32 seq, HV *hh)
             SvREFCNT_dec(GvHV(PL_hintgv));
             GvHV(PL_hintgv) = hh;
             FETCHFEATUREBITSHH(hh);
+            SV *versv, **svp;
+            if((svp = hv_fetchs(hh, "CORE/prevailing_version", 0)) && (versv = *svp) && SvOK(versv)) {
+                SAVEI16(PL_prevailing_version);
+                PL_prevailing_version = SvUV(versv);
+            }
         }
     }
     SAVECOMPILEWARNINGS();

t/lib/test_22121.pm

@@ -0,0 +1,2 @@
+eval "use v5.14;";
+1;

t/lib/warnings/op

@@ -2180,3 +2180,9 @@ use v5.12;
 use v5.12;
 # expect no warning because same version
 EXPECT
+########
+# GH #22121
+use v5.20;
+use test_22121;
+# expect no warning because different scope
+EXPECT