in reply to how can I combine these expressions?

How about this:
$str =~ s/(^|[^\%](?:\%\%)*)\% (\{)? # Match and capture 1 or 0 braces ([_a-zA-Z]\w*) (?(2)\}) # If there's anything in $2, match en +ding brace /"$1".$env->{$3}/gex; # $3 now instead of $2 here
This takes advantage of perl's conditional matching operator. From perlre:
(?(condition)yes-pattern|no-pattern) (?(condition)yes-pattern) Conditional expression. (condition) should be either an integer in parentheses (which is valid if the corresponding pair of parentheses matched), or lookahead/lookbehind/evaluate zero-width assertion. Say, m{ ( \( )? [^()]+ (?(1) \) ) }x matches a chunk of non-parentheses, possibly included in parentheses themselves.