use strict;

my $a = '(0,((1,2),3,(4,5),6),7,8)';
my $tail = qr{5\)};
my $matching_parens = qr{
    (                         # paren group 1 (parens)
        \(
            (?:
                (?> [^()]+ )  # Non-parens without backtracking
                |
                (?1)          # Recurse to start of paren group 1
            )*
        \)
    )
    .*
    (?<= $tail )
}x;
$a =~ /$matching_parens/;
print "$1\n";