fix false positive on attempt to escape whitespace in qw()

..originally added in GH #23403.

Author: karenetheridge

t/lib/warnings/toke

@@ -375,6 +375,33 @@ EXPECT
 Possible attempt to escape whitespace in qw() list at - line 3.
 ########
 # toke.c
+use warnings 'qw';
+@a = qw( foo bar \ baz );
+EXPECT
+Possible attempt to escape whitespace in qw() list at - line 3.
+########
+# toke.c
+use warnings 'qw';
+@a = qw(\ );
+EXPECT
+Possible attempt to escape whitespace in qw() list at - line 3.
+########
+# toke.c
+use warnings 'qw';
+@a = qw(\\ );
+EXPECT
+########
+# toke.c
+use warnings 'qw';
+@a = qw( \\ );
+EXPECT
+########
+# toke.c
+use warnings 'qw';
+@a = qw( foo bar \\ baz );
+EXPECT
+########
+# toke.c
 use warnings 'syntax' ;
 print ("");
 print ("") and $x = 1;

toke.c

@@ -5820,13 +5820,14 @@ yyl_qw(pTHX_ char *s, STRLEN len)
         int warned_comment = warned_comma;
         int warned_escape = warned_comma;
         char *d = SvPV_force(PL_lex_stuff, len);
+        const STRLEN orig_len = len;
         while (len) {
             for (; isSPACE(*d) && len; --len, ++d)
                 /**/;
             if (len) {
                 SV *sv;
                 const char *b = d;
-                if (!warned_comma || !warned_comment) {
+                if (!warned_comma || !warned_comment || !warned_escape) {
                     for (; !isSPACE(*d) && len; --len, ++d) {
                         if (!warned_comma && *d == ',') {
                             warner(packWARN(WARN_QW),
@@ -5838,7 +5839,8 @@ yyl_qw(pTHX_ char *s, STRLEN len)
                                    "Possible attempt to put comments in qw() list");
                             ++warned_comment;
                         }
-                        else if (!warned_escape && *d == '\\' && len > 1 && isSPACE(*(d+1)) ) {
+                        else if (!warned_escape && *d == '\\' && len > 1 && isSPACE(*(d+1))
+                                 && (len == orig_len || *(d-1) != '\\')) {
                             warner(packWARN(WARN_QW),
                                    "Possible attempt to escape whitespace in qw() list");
                             ++warned_escape;