$$x is ${'$y'}, and '$y' is in the main symbol table:

  DB<1> l
2==>       $y = 'bob';
3:         $x = '$y';
4:         $$x = 'wierd';
5       
6:         print $$x;    # prints 'wierd' ($$x has a value)
7:         print ' - ';  # print a little seperator
8:         print $bob;   # prints nothing ($bob is still undefined)
  DB<1> n
main::(-:3):       $x = '$y';
  DB<1> n
main::(-:4):       $$x = 'wierd';
  DB<1> n
main::(-:6):       print $$x;    # prints 'wierd' ($$x has a value)
  DB<1> X ~y
$$y = 'wierd'
$y = 'bob'
  DB<2> x ${$::{'$y'}}
0  'wierd'
  DB<3>
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(Remember that $x = '$y' is very different from $x = "$y".)

Update: Double quotes in the first paragraph changed to single quotes.