Revert "[perl #119043] Exempt shared hash key consts from ro"

Revert "[perl #119043] Exempt shared hash key consts from ro"

This reverts commit ba36554e02872e48d146177a57a9cfb154727fae.

It turns out it reinstates bugs like this:

$ perl -e 'for(1..10){for(__PACKAGE__){warn $_; $_++}}'
main at -e line 1.
maio at -e line 1.
maip at -e line 1.
maiq at -e line 1.
mair at -e line 1.
mais at -e line 1.
mait at -e line 1.
maiu at -e line 1.
maiv at -e line 1.
maiw at -e line 1.