Add "\" to characters that produce warnings in qw().

Perl 5.43.2 added diagnostic `Possible attempt to escape whitespace in
qw() list`, which triggers on the detection of a reverse solidus. This
commit rewrites the last paragraph in the section on `qw()` to add the
reverse solidus to the list of characters that produce a warning.

Author: trwyant

pod/perlop.pod

@@ -2089,10 +2089,11 @@ Some frequently seen examples:
     use POSIX qw( setlocale localeconv )
     @EXPORT = qw( foo bar baz );
 
-A common mistake is to try to separate the words with commas or to
-put comments into a multi-line C<qw>-string.  For this reason, the
-S<C<use warnings>> pragma and the B<-w> switch (that is, the C<$^W> variable)
-produces warnings if the I<STRING> contains the C<","> or the C<"#"> character.
+Common mistakes are trying to separate the words with commas, trying to
+put comments into a multi-line C<qw>-string, or trying to C<\>-escape
+the space between words. For this reason, the S<C<use warnings>> pragma
+and the B<-w> switch (that is, the C<$^W> variable) produce warnings if
+the I<STRING> contains the C<",">, C<"#">, or C<"\"> characters.
 
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