uninit warning from $h{\const} coredumped

The code that printed the the name and subscript of a hash element
in an "uninitialized variable" warning assumed that a constant
hash subscript would be SvPOK. Something like \1 is a constant,
but is ROK, not POK. SEGVs ensured.

sv.c

@@ -15683,9 +15683,12 @@ Perl_varname(pTHX_ const GV *const gv, const char gvtype, PADOFFSET targ,
 
     if (subscript_type == FUV_SUBSCRIPT_HASH) {
 	SV * const sv = newSV(0);
+        STRLEN len;
+        const char * const pv = SvPV_nomg_const((SV*)keyname, len);
+
 	*SvPVX(name) = '$';
 	Perl_sv_catpvf(aTHX_ name, "{%s}",
-	    pv_pretty(sv, SvPVX_const(keyname), SvCUR(keyname), 32, NULL, NULL,
+	    pv_pretty(sv, pv, len, 32, NULL, NULL,
 		    PERL_PV_PRETTY_DUMP | PERL_PV_ESCAPE_UNI_DETECT ));
 	SvREFCNT_dec_NN(sv);
     }

t/op/hashwarn.t

@@ -6,7 +6,7 @@ BEGIN {
 }
 
 require './test.pl';
-plan( tests => 16 );
+plan( tests => 18 );
 
 use strict;
 use warnings;
@@ -71,3 +71,20 @@ my $fail_not_hr   = 'Not a HASH reference at ';
     cmp_ok(scalar(@warnings),'==',0,'pseudo-hash 2 count');
     cmp_ok(substr($@,0,length($fail_not_hr)),'eq',$fail_not_hr,'pseudo-hash 2 msg');
 }
+
+# RT #128189
+# this used to coredump
+
+{
+    @warnings = ();
+    my %h;
+
+    no warnings;
+    use warnings qw(uninitialized);
+
+    my $x = "$h{\1}";
+    is(scalar @warnings, 1, "RT #128189 - 1 warning");
+    like("@warnings",
+        qr/Use of uninitialized value \$h\{"SCALAR\(0x[\da-f]+\)"\}/,
+        "RT #128189 correct warning");
+}