Fails on the perfectly balanced "12(34)56(78)9"

Assuming it's about removing balanced parenthesis, I'd go for:

my $balance;
# Notice declaring $balance beforehand is important, 
# otherwise you pick up the global inside (??{})
$balance=qr/(?:[^()]|\((??{$balance})\))*/;
$string =~ s/\($balance\)//g;
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