Forbid any downgrade of use VERSION once at least v5.39 on account of…

… imported builtin lexicals

op.c

@@ -8046,6 +8046,14 @@ Perl_utilize(pTHX_ int aver, I32 floor, OP *version, OP *idop, OP *arg)
                 PL_hints &= ~HINT_STRICT_VARS;
         }
 
+        /* Forbid any VERSION downgrade when already above 5.39 because doing
+         * so risks disabling an imported builtin, and we don't (currently)
+         * have an ability to do this.
+         */
+        if (PL_prevailing_version >= SHORTVER(5, 39) && shortver < PL_prevailing_version) {
+            croak("Downgrading a use VERSION declaration when above v5.39 is not permitted");
+        }
+
         if (shortver >= SHORTVER(5, 39)) {
             prepare_export_lexical();
             import_builtin_bundle(shortver);

pod/perldiag.pod

@@ -2234,6 +2234,25 @@ declaration of one having a larger number (which would have enabled
 these effects). Because of a change to the way that C<use VERSION>
 interacts with the strictness flags, this is no longer supported.
 
+=item Downgrading a use VERSION declaration when above v5.39 is not permitted
+
+(F) An attempt was made to back-pedal the prevailing C<use VERSION> to a
+lower number, while the version is at least v5.39.  Because any version at
+least v5.39 imports new lexical symbols from L<builtin>, such imports would
+need removing from the scope.  At present there is no facility for doing
+this, so such version downgrades are not permitted.
+
+Instead, if you need to use a specifically later version only in a small
+scope, use a nested block.  Upon leaving the block afterwards, all of the
+lexical state will be restored - including removing the extra imported
+symbols.
+
+    use v5.20;
+    {
+	use v5.39;
+	# extra symbols can be used in here
+    }
+
 =item (Do you need to predeclare %s?)
 
 (S syntax) This is an educated guess made in conjunction with the message

pod/perlfunc.pod

@@ -10183,6 +10183,21 @@ file (possibly after a C<package> statement or any amount of whitespace or
 comment), so that its effects happen first, and other pragmata are applied
 after it.
 
+Furthermore, once the version is 5.39.0 or higher, no later C<use VERSION>
+declaration within the same scope is permitted to lower the version again.
+This is because any imported functions from C<builtin> would need to be made
+invisible again.  If you wish to use a higher version only in a small
+section of a file, enclose that section in its own lexical scope.  Once
+outside the scope again afterwards, any imported effects are no longer
+visible.
+
+    use v5.20;
+    {
+	use v5.39;
+	# imported builtins are now visible
+    }
+    # any effects of the block above have now been removed
+
 Specifying VERSION as a numeric argument of the form 5.024001 should
 generally be avoided as older less readable syntax compared to
 v5.24.1. Before perl 5.8.0 released in 2002 the more verbose numeric

t/comp/use.t

@@ -6,7 +6,7 @@ BEGIN {
     $INC{"feature.pm"} = 1; # so we don't attempt to load feature.pm
 }
 
-print "1..87\n";
+print "1..89\n";
 
 # Can't require test.pl, as we're testing the use/require mechanism here.
 
@@ -157,6 +157,12 @@ like $@, qr/^Can't use string/,
     like $warnings, qr/^Downgrading a use VERSION declaration to below v5.11 is deprecated, and will become fatal in Perl 5.40 at /,
         'use 5.01 after use 5.012 provokes deprecation warning';
 }
+eval 'use 5.039; use 5.020;';
+like $@, qr/^Downgrading a use VERSION declaration when above v5\.39 is not permitted at /,
+    'downgrading prevailing version in same scope is not permitted';
+eval '{ use 5.039; } use 5.020;';
+ok !$@, 'downgrading prevailing version by falling out of scope is permitted';
+
 eval 'no strict "subs"; use 5.012; ${"foo"} = "bar"';
 ok $@, 'no strict subs allows ver decl to enable refs';
 eval 'no strict "subs"; use 5.012; $nonexistent_pack_var';