$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
##```
##
(?(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.
```