I would recommend using the CPAN module as suggested by larsen. However I am presenting this solution just for fun... :-)

use strict;
use warnings;

my $str = 'I am (giving this (text to (tell you) that this) will give)
+ you the result)))';

my $match = '';

{
    local $_ = $str;
    my $n = 0;

    s/([()]|.)/
      if ($1 eq '(') {
            $n++;
            $match .= $1
      } elsif ($1 eq ')') {
            if ($n) {
                $n--;
                $match .= $1
            }
      } else {
            $match .= $1 if $n
      }
    /gex;
}

print "$match\n";

__END__

# output
(giving this (text to (tell you) that this) will give)
[download]