1. You are assigning an array to a scalar ($string = (@_)). Value of $string becomes a numeric representing the number of elements in @_
2. $testString does not seem to be initialized
3. You need not specify single quotes to enclose brackets. Escaping is enough.
4. In your example only those brackets will be considered which have an digit following immediately.

Depending on how complex you want to be, the program may require many changes but taking the simplest approach, I think following will suit your requirements.

#!/usr/bin/perl
use strict;
sub parenth {
        my ($string) = (@_);

        my $l_count = () = ($string =~ /\(/g);
        my $r_count = () = ($string =~ /\)/g);
        if ($l_count != $r_count) {
                print "There is no balance on this line ($l_count vs $
+r_count)! Did you format it correctly???";
                return 0;
        }
        else {
                print "There is balance in this universe";
                return 1;
        }
}


my $test_string = 'x = (89 - 12)*( 1 + 2)';
parenth($test_string);
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