Of course, that leaves the closing ')' in place :)

Text::Balanced might be an option:

$ perl -e "
use strict;
use Text::Balanced qw/extract_bracketed/;
my $var='Aur S-P 250 Pre (Chlorine + cillin (cilli G caine) + Sulfate)
+';
my ($ext,$rem,$pre)=extract_bracketed($var,'()',qr/[^(]+/);
die 'oops' if $rem;
$ext =~ s/(^\(|\)$)//sg; # remove lead/trail ()
print qq{$pre\n$ext\n};
"
Aur S-P 250 Pre
Chlorine + cillin (cilli G caine) + Sulfate
[download]