[perl #92446] don't use overloaded string values in the backtrace

Allowing string overloading caused some CPAN module tests to fail.

While it seems reasonable to allow "" overloading for the backtrace, it has
several problems:

- the overloaded stringification may be suitable for end-users, but not
  for debugging, the raw reference display at least allows for identifying
  which object is involved

- the overload may not be suitable for calling in an exception, eg. it may
  call parts of Carp itself internally, or throw its own exception

- the overload may not be intended for use, eg. deprecation

This changes fixes the failures for Data-Hive and Contextual-Return
reported in [perl #119321]