use strict;
use warnings;

my $string = 'this is (testing 123(test%^*&()()
fghfgh(in)))*&^@#%$))))((   (((
(g) 123)a test';

$string =~ s/\(.*\)//gs;
print $string;
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Updated: Swapped to /s modifier to allow for multiple lines.
UPDATE 2: Added /g modifier back into regex.