$s = join('|',
   map { local $_=$_; s/\^/^1/g; s/\|/^2/g; $_ }
      %test_hash
);

##</code><code>##

%test_hash =  map { local $_=$_; s/\^1/^/g; s/\^2/|/g; $_ }
   split(/\|/, $s);

##</code><code>##

use Data::Dumper ();

%test_hash = (
  'apple' => 'red',
  'lime'  => 'green',
  'junk'  => 'foo^bar',
  'junk2' => 'bla|bla',
);

print(Data::Dumper->Dump([ \%test_hash ],[qw( $test_hash_ref )]));
print("\n");

$s = join('|',
   map { local $_=$_; s/\^/^1/g; s/\|/^2/g; $_ }
      %test_hash
);

print($s, "\n");
print("\n");

%test_hash =  map { local $_=$_; s/\^1/^/g; s/\^2/|/g; $_ }
   split(/\|/, $s);

print(Data::Dumper->Dump([ \%test_hash ],[qw( $test_hash_ref )]));

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

apple|red|junk|foo^1bar|junk2|bla^2bla|lime|green

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