Add TODO test for detecting tied packages

Currently, a package is believed to be unused if it is only used in a `tie` statement.

t/tie.t

@@ -0,0 +1,29 @@
+use strict;
+use warnings;
+
+use lib 't/lib';
+
+use TestHelper qw( doc );
+use Test::More import => [qw( done_testing is $TODO )];
+
+my $test_data_filename = 'test-data/tie.pl';
+my ($doc) = doc(
+    filename => $test_data_filename,
+    preserve_unused => 0,
+);
+
+open my $test_data, '<', $test_data_filename
+    or die "Could not open $test_data_filename: $!";
+my $expected = do {
+    local $/;
+    <$test_data>;
+};
+close $test_data
+    or die "Error on closing $test_data_filename: $!";
+
+TODO: {
+    local $TODO = 'Cannot yet see packages used by tie';
+    is( $doc->tidied_document, $expected, 'tie observed' );
+}
+
+done_testing();

test-data/tie.pl

@@ -0,0 +1,6 @@
+use strict;
+use warnings;
+
+use Tie::SubstrHash ();
+
+tie my %hash, 'Tie::SubstrHash', 1, 1, 1;