[perl #96230] Stop qr// from reusing last pattern

qr// should not be using the last-successful pattern, because it is
"(?^:)", not the empty pattern.  A stringified qr// does not use the
last-successful pattern.

Author: Father Chrysostomos

pp_hot.c

@@ -1286,8 +1286,10 @@ PP(pp_match)
 
 
 
-    /* empty pattern special-cased to use last successful pattern if possible */
-    if (!RX_PRELEN(rx) && PL_curpm) {
+    /* empty pattern special-cased to use last successful pattern if
+       possible, except for qr// */
+    if (!((struct regexp *)SvANY(rx))->mother_re && !RX_PRELEN(rx)
+     && PL_curpm) {
 	pm = PL_curpm;
 	rx = PM_GETRE(pm);
     }

t/op/qr.t

@@ -7,7 +7,7 @@ BEGIN {
     require './test.pl';
 }
 
-plan(tests => 18);
+plan(tests => 19);
 
 sub r {
     return qr/Good/;
@@ -59,3 +59,7 @@ $$e = 'Fake!';
 is($$e, 'Fake!');
 object_ok($e, 'Stew');
 like("$e", qr/\Stew=SCALAR\(0x[0-9a-f]+\)\z/);
+
+# [perl #96230] qr// should not have the reuse-last-pattern magic
+"foo" =~ /foo/;
+like "bar",qr//,'[perl #96230] =~ qr// does not reuse last successful pat';