my $regex = qr /(              # Start capture
                \(              # Start with '(',
                (?:             # Followed by
                (?>[^()]+)      # Non-parenthesis
                |(??{ $regex }) # Or a balanced () block
                )*              # zero or more times
                \)              # Close capture
                )/x;            # Ending with ')'

##</code><code>##

my $regex = qr /(              # Start capture
                \(              # Start with '(',
                (?:             # Followed by
                (?>[^()]+)      # Non-parenthesis
                |(??{ undef })  # <-- Note! 'or undef'
                )*              # zero or more times
                \)              # Close capture
                )/x;            # Ending with ')'