for (
    'oh {hello} there',
    'oh {hello there',
    'oh hello there',
    'oh hello} there',
) {
    our $paren;
    if (/
         ({ | (?<!{))                   # optional opening brace
         (?{ $paren = $^N })            # store for later
         hello                          # .. followed by 'hello'
         (??{$paren ? "\}" : "(?!\})"}) # a closing brace if the open brace was there
        /x) {
        print "Yep - $1\n";
    } else {
        print "Nope\n";
    }
}