The code above only covers array and hashes of strings, and undef is not a string. Below is a variation that supports undef. Anything more complicated should definitely use FreezeThaw.

sub serialize_string_list {
   return join('|',
      map {
         (defined($_)
            ? do { local $_=$_; s/\^/^1/g; s/\|/^2/g; $_ }
            : '^0'
         )
      } @_
   );
}

sub deserialize_string_list {
   return map {
      ($_ eq '^0'
         ? undef
         : do { local $_=$_; s/\^1/^/g; s/\^2/|/g; $_ }
      )
   } split(/\|/, $_[0]);
}

$s = serialize_string_list(%test_hash);
%test_hash = deserialize_string_list($s);
[download]

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

Use of uninitialized value in substitution (s///) at ./serialize.pl li
+ne 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' => ''
                 };
[download]

CT