my $bal = # this is from perlre
qr/
    \(
    (?:
        (?> [^()]+ )
        |
        (??{$bal})
    )*
    \)
/x;
$text =~
s/
    (
        (?:
            (??{$bal})
            [^(\d]*
        )*
    )
    (\d+)
    (?= \) )
/$1\n$2/xg;