Something that hasn't being mentioned is that all punctuation variables are always global, even if they aren't magical. It can be hard to find unused punctuation characters though!

  %% = (foo => 23);
  package xxx;
  print $%{foo};
[download]

I dimly remember that in Perl 4 any variable or function starting with an underscore was always global; but that changed when Perl 5 came along.

The situation with ${^Name} variables is a little more complicated: the fact is that any variable name that starts with a punctuation character is implicitly global, but the Perl parser only recognises single punctuation characters. There's one exception to that: you can also use :: as a name, so you can use the variable $:: which is of course global. (And no, it's not a package qualifier - the variable really is called ::. The reason it's allowed is because the root symbol table hash is called %::, so the variable name tokeniser needs to let it through.) But underneath, any string of characters which begins with punctuation will force globality:

  ${'@$??!'} = "Weird!\n";
  package SomewhereElse;
  print ${'@$??!'}
[download]

What's special about ${^Thing} is that (in recent releases) the parser is able to parse the name directly, and it converts the first character into a control char! So the following will work, because control-H is the backspace character:

  ${^Hello} = "Curious...\n";
  print ${"\bello"};
[download]

Some food for obfu, maybe... :)