When you get warnings you do not understand, try using the diagnostics module:

\1 better written as $1 at - line 3 (#1)
  (W syntax) Outside of patterns, backreferences live on as variables.
  The use of backslashes is grandfathered on the right-hand side of a
  substitution, but stylistically it's better to use the variable form
  because other Perl programmers will expect it, and it works better i
+f
  there are more than 9 backreferences.
[download]