my $example1 = 'this(is(a(test)))';

foreach (0..4) {
	print "level $_: '", peel($example1, $_), "'\n";
}
print "level -1: '", peel($example1, -1), "'\n";

sub peel {
	my ($str, $level) = @_;
	if ($level < 0) {
		$str =~ /([^\(\)]+)\)+\Z/;
		return $1;
	}
	my $opening = '(?:[^\(]+\(){' . $level . '}';
	my $closing = '\){' . $level . '}';
	$str =~ /\A$opening(.+?)$closing\Z/;
	return $1;
}