$test_hash_ref = {
                   'apple' => 'red',
                   'junk' => 'foo^bar',
                   'junk2' => undef,
                   'lime' => 'green'
                 };

Use of uninitialized value in substitution (s///) at ./serialize.pl line 19.
Use of uninitialized value in join or string at ./serialize.pl line 19.
apple|red|junk|foo^1bar|junk2||lime|green

$test_hash_ref = {
                   'apple' => 'red',
                   'junk' => 'foo^bar',
                   'lime' => 'green',
                   'junk2' => ''
                 };