# This works fine
$a = "abc";
$a =~ s/^(?:a(\w))?(\w)$/$1:$2/; # works fine

# This produces the warning
# "Use of uninitialized value in concatenation (.)
$b = "c";
$b =~ s/^(?:a(\w))?(\w)$/$1:$2/;

##</code><code>##

$b =~ s/^a?(\w?)(\w)$/$1:$2/; # no warning here.