diff --git a/sv.h b/sv.h
index 449091e85375..05a83638cacc 100644
--- a/sv.h
+++ b/sv.h
@@ -1590,9 +1590,19 @@ L</C<SV_CHECK_THINKFIRST_COW_DROP>> before calling this.
 =cut
 */
 
-#define SvPV_shrink_to_cur(sv) STMT_START { \
-                   const STRLEN _lEnGtH = SvCUR(sv) + 1; \
-                   SvPV_renew(sv, _lEnGtH); \
+/* Notes: Ensure the buffer is big enough to be COWed in the future, so
+          + 1 for the trailing null byte + 1 for the COW count.
+ * Don't try to shrink if the saving is too small to be realistic (given
+ * the realities of allocators) or noticeable.
+ * Don't try to shrink smaller than PERL_STRLEN_NEW_MIN, as that is
+ * unlikely to result in a smaller allocation.
+*/
+
+#define SvPV_shrink_to_cur(sv) STMT_START {                \
+                   const STRLEN _lEnGtH = SvCUR(sv) + 2;   \
+                   if ((SvLEN(sv) > _lEnGtH + PTRSIZE) &&  \
+                       ( _lEnGtH >= PERL_STRLEN_NEW_MIN) ) \
+                       SvPV_renew(sv, _lEnGtH);            \
                  } STMT_END
 
 /*
diff --git a/toke.c b/toke.c
index 5759c7890d32..671c4af209a2 100644
--- a/toke.c
+++ b/toke.c
@@ -4422,9 +4422,7 @@ S_scan_const(pTHX_ char *start)
     }
 
     /* shrink the sv if we allocated more than we used */
-    if (SvCUR(sv) + 5 < SvLEN(sv)) {
-        SvPV_shrink_to_cur(sv);
-    }
+    SvPV_shrink_to_cur(sv);
 
     /* return the substring (via pl_yylval) only if we parsed anything */
     if (s > start) {
@@ -11348,9 +11346,7 @@ S_scan_heredoc(pTHX_ char *s)
         SvREFCNT_dec_NN(newstr);
     }
 
-    if (SvCUR(tmpstr) + 5 < SvLEN(tmpstr)) {
-        SvPV_shrink_to_cur(tmpstr);
-    }
+    SvPV_shrink_to_cur(tmpstr);
 
     if (!IN_BYTES) {
         if (UTF && is_utf8_string((U8*)SvPVX_const(tmpstr), SvCUR(tmpstr)))
@@ -11877,10 +11873,7 @@ Perl_scan_str(pTHX_ char *start, int keep_bracketed_quoted, int keep_delims, int
     PL_parser->herelines = herelines;
 
     /* if we allocated too much space, give some back */
-    if (SvCUR(sv) + 5 < SvLEN(sv)) {
-        SvLEN_set(sv, SvCUR(sv) + 1);
-        SvPV_shrink_to_cur(sv);
-    }
+    SvPV_shrink_to_cur(sv);
 
     /* decide whether this is the first or second quoted string we've read
        for this op