From 2c32e432874f2da0cf2cfe63cd181f7e3b1ae103 Mon Sep 17 00:00:00 2001
From: "Philippe Bruhat (BooK)" <book@cpan.org>
Date: Fri, 4 Jul 2025 19:30:32 +0200
Subject: [PATCH 1/2] add a new warning against using \ in qw()

I've seen AI-generated code try to use qw() to create lists containing
strings with embedded whitespace using qw and \ to "protect" the
whitespace. Things like:

    my @list = qw(
        foo
        bar\ baz
    );

Just like occurences of ',' and '#', I believe this should warn.

Note that the warning will only be emitted when the \ is followed by
actual whitespace, so code like the following (from lib/App/Cpan.pm)
will not warn:

    my $epic_fail_words = join '|',
            qw( Error stop(?:ping)? problems force not unsupported
                    fail(?:ed)? Cannot\s+install );
---
 pod/perldiag.pod    | 27 +++++++++++++++++++++++++++
 t/lib/warnings/toke |  9 +++++++++
 toke.c              |  6 ++++++
 3 files changed, 42 insertions(+)

diff --git a/pod/perldiag.pod b/pod/perldiag.pod
index 6c9948f861e9..7d28d1dc75c6 100644
--- a/pod/perldiag.pod
+++ b/pod/perldiag.pod
@@ -5467,6 +5467,33 @@ character class, just escape the square brackets with the backslash: "\[="
 and "=\]".  The S<<-- HERE> shows whereabouts in the regular expression the
 problem was discovered.  See L<perlre>.
 
+=item Possible attempt to escape whitespace in qw() list
+
+(W qw) qw() lists contain items separated by whitespace; contrary to
+what some might expect, backslash characters cannot be used to "protect"
+whitespace from being split, but are instead treated as literal data.
+(You may have used different delimiters than the parentheses shown here;
+braces are also frequently used.)
+
+You probably wrote something like this:
+
+    @list = qw(
+        a\ string
+        another
+    );
+
+Expecting to get a two elements list containing the strings C<'a string'>
+and C<'another'>.  Instead the list will hold four elements: C<'a\'>
+(with a literal backslash), C<'string'> and C<'another'>.
+
+If you really want whitespace in your strings, build your list the
+old-fashioned way, with quotes and commas:
+
+    @list = ( 'a string', 'another' );
+
+Note that this warnings is I<only> emitted when the backslash is followed
+by actual whitespace (that C<qw> splits on).
+
 =item Possible attempt to put comments in qw() list
 
 (W qw) qw() lists contain items separated by whitespace; as with literal
diff --git a/t/lib/warnings/toke b/t/lib/warnings/toke
index 6938174544fd..b3ade75122c5 100644
--- a/t/lib/warnings/toke
+++ b/t/lib/warnings/toke
@@ -49,6 +49,9 @@ toke.c	AOK
      Possible attempt to put comments in qw() list 
 	@a = qw(a b # c) ;
 
+     Possible attempt to escape whitespace in qw() list
+	@a = qw( foo bar\ baz ) ;
+
      %s (...) interpreted as function 
 	print ("")
 	printf ("")
@@ -366,6 +369,12 @@ Possible attempt to separate words with commas at - line 3.
 Possible attempt to put comments 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 'syntax' ;
 print ("");
 print ("") and $x = 1;
diff --git a/toke.c b/toke.c
index 5759c7890d32..57ea5c5e4810 100644
--- a/toke.c
+++ b/toke.c
@@ -5821,6 +5821,7 @@ yyl_qw(pTHX_ char *s, STRLEN len)
     if (SvCUR(PL_lex_stuff)) {
         int warned_comma = !ckWARN(WARN_QW);
         int warned_comment = warned_comma;
+        int warned_escape = warned_comma;
         char *d = SvPV_force(PL_lex_stuff, len);
         while (len) {
             for (; isSPACE(*d) && len; --len, ++d)
@@ -5840,6 +5841,11 @@ 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)) ) {
+                            warner(packWARN(WARN_QW),
+                                   "Possible attempt to escape whitespace in qw() list");
+                            ++warned_escape;
+                        }
                     }
                 }
                 else {

From ee06f66cffe6d0c8442b7e1b6c580d853768f3e8 Mon Sep 17 00:00:00 2001
From: "Philippe Bruhat (BooK)" <book@cpan.org>
Date: Tue, 8 Jul 2025 18:30:41 +0200
Subject: [PATCH 2/2] add perldelta entry for the new qw warning

---
 pod/perldelta.pod | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/pod/perldelta.pod b/pod/perldelta.pod
index 1b80c902c3d0..ea5770a8e27b 100644
--- a/pod/perldelta.pod
+++ b/pod/perldelta.pod
@@ -206,7 +206,14 @@ XXX L<message|perldiag/"message">
 
 =item *
 
-XXX L<message|perldiag/"message">
+L<Possible attempt to escape whitespace in qw() list|perldiag/"Possible attempt to escape whitespace in qw() list">
+
+(W qw) qw() lists contain items separated by whitespace; contrary to
+what some might expect, backslash characters cannot be used to "protect"
+whitespace from being split, but are instead treated as literal data.
+
+Note that this warnings is I<only> emitted when the backslash is followed
+by actual whitespace (that C<qw> splits on).
 
 =back