use strict;

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

my $text = '(outer(inner(most inner)))';

$text =~ /$regex/gs;

print "$1\n";