You missed this part of perlthrtut:

In the case of a shared array, all the array's elements are shared,
and for a shared hash, all the keys and values are shared.
This places restrictions on what may be assigned to shared
array and hash elements:
only simple values or references to shared variables are
allowed - this is so that a private variable can't
accidentally become shared. A bad assignment will cause
the thread to die.
[download]

Your hash values are neither simple nor shared.