$str =~ s/(^|[^\%](?:\%\%)*)\%
          (\{)?                  # Match and capture 1 or 0 braces
          ([_a-zA-Z]\w*)
          (?(2)\})               # If there's anything in $2, match ending brace
          /"$1".$env->{$3}/gex;  # $3 now instead of $2 here