Add a PL_prevailing_version interpreter var

Save/restore PL_prevailing_version at SAVEHINTS time

Have PL_prevailing_version track the applied use VERSION currently in scope

embedvar.h

@@ -233,6 +233,7 @@
 #define PL_phase		(vTHX->Iphase)
 #define PL_pidstatus		(vTHX->Ipidstatus)
 #define PL_preambleav		(vTHX->Ipreambleav)
+#define PL_prevailing_version	(vTHX->Iprevailing_version)
 #define PL_profiledata		(vTHX->Iprofiledata)
 #define PL_psig_name		(vTHX->Ipsig_name)
 #define PL_psig_pend		(vTHX->Ipsig_pend)

intrpvar.h

@@ -1029,6 +1029,12 @@ PERLVAR(I, wcrtomb_ps, mbstate_t)
 PERLVARA(I, mem_log, 1 + 1 + TYPE_DIGITS(UV) + 1 + 3 + 1, char)
 #endif
 
+/* The most recently seen `use VERSION` declaration, encoded in a single
+ * U16 as (major << 8) | minor. We do this rather than store an entire SV
+ * version object so we can fit the U16 into the uv of a SAVEHINTS and not
+ * have to worry about SV refcounts during scope enter/exit. */
+PERLVAR(I, prevailing_version, U16)
+
 /* If you are adding a U8 or U16, check to see if there are 'Space' comments
  * above on where there are gaps which currently will be structure padding.  */
 

op.c

@@ -8842,6 +8842,32 @@ Perl_package_version( pTHX_ OP *v )
     op_free(v);
 }
 
+/* grrrr */
+static U16 S_extract_shortver(pTHX_ SV *sv)
+{
+    SV *rv;
+    if(!SvRV(sv) || !SvOBJECT(rv = SvRV(sv)) || !sv_derived_from(sv, "version"))
+        return 0;
+
+    AV *av = MUTABLE_AV(SvRV(*hv_fetchs(MUTABLE_HV(rv), "version", 0)));
+
+    U16 shortver = 0;
+
+    IV major = av_count(av) > 0 ? SvIV(*av_fetch(av, 0, false)) : 0;
+    if(major > 255)
+        shortver |= 255 << 8;
+    else
+        shortver |= major << 8;
+
+    IV minor = av_count(av) > 1 ? SvIV(*av_fetch(av, 1, false)) : 0;
+    if(minor > 255)
+        shortver |= 255;
+    else
+        shortver |= minor;
+
+    return shortver;
+}
+
 void
 Perl_utilize(pTHX_ int aver, I32 floor, OP *version, OP *idop, OP *arg)
 {
@@ -8926,6 +8952,8 @@ Perl_utilize(pTHX_ int aver, I32 floor, OP *version, OP *idop, OP *arg)
         use_version = sv_2mortal(new_version(use_version));
         S_enable_feature_bundle(aTHX_ use_version);
 
+        U16 shortver = S_extract_shortver(aTHX_ use_version);
+
         /* If a version >= 5.11.0 is requested, strictures are on by default! */
         if (vcmp(use_version,
                  sv_2mortal(upg_version(newSVnv(5.011000), FALSE))) >= 0) {
@@ -8948,6 +8976,8 @@ Perl_utilize(pTHX_ int aver, I32 floor, OP *version, OP *idop, OP *arg)
             if (!(PL_hints & HINT_EXPLICIT_STRICT_VARS))
                 PL_hints &= ~HINT_STRICT_VARS;
         }
+
+        PL_prevailing_version = shortver;
     }
 
     /* The "did you use incorrect case?" warning used to be here.

pp_ctl.c

@@ -3539,6 +3539,7 @@ S_doeval_compile(pTHX_ U8 gimme, CV* outside, U32 seq, HV *hh)
     SAVEHINTS();
     if (clear_hints) {
         PL_hints = HINTS_DEFAULT;
+        PL_prevailing_version = 0;
         hv_clear(GvHV(PL_hintgv));
         CLEARFEATUREBITS();
     }

scope.c

@@ -699,14 +699,14 @@ Perl_save_hints(pTHX)
             SS_ADD_INT(PL_hints);
             SS_ADD_PTR(save_cophh);
             SS_ADD_PTR(oldhh);
-            SS_ADD_UV(SAVEt_HINTS_HH);
+            SS_ADD_UV(SAVEt_HINTS_HH | (PL_prevailing_version << 8));
             SS_ADD_END(4);
         }
         GvHV(PL_hintgv) = NULL; /* in case copying dies */
         GvHV(PL_hintgv) = hv_copy_hints_hv(oldhh);
         SAVEFEATUREBITS();
     } else {
-        save_pushi32ptr(PL_hints, save_cophh, SAVEt_HINTS);
+        save_pushi32ptr(PL_hints, save_cophh, SAVEt_HINTS | (PL_prevailing_version << 8));
     }
 }
 
@@ -1380,6 +1380,7 @@ Perl_leave_scope(pTHX_ I32 base)
             cophh_free(CopHINTHASH_get(&PL_compiling));
             CopHINTHASH_set(&PL_compiling, (COPHH*)a1.any_ptr);
             *(I32*)&PL_hints = a0.any_i32;
+            PL_prevailing_version = (U16)(uv >> 8);
             if (type == SAVEt_HINTS_HH) {
                 SvREFCNT_dec(MUTABLE_SV(GvHV(PL_hintgv)));
                 GvHV(PL_hintgv) = MUTABLE_HV(a2.any_ptr);