While there are some issues with your chosen method of iterating (not the least of which is $key will be set to both the keys and
the values of the hash, since foreach puts the hash in list context and thus flattens it), there is no
circumstance where you should get an attempt to free unreferenced scalar
error. That's an indication of an internal error, and you'll only get that when something goes wrong inside perl. It's normally an indication that an XS module has an error in it, but in this case you seem to be tripping a perl bug.
File a bug with the perl5-porters (use the perlbug command installed with perl) so someone can take a look at this and figure out what's wrong.