Test that __PACKAGE__ is read-only

Test that __PACKAGE__ is read-only

I.e., test that this bug is fixed:

$ 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.

for my$p(__PACKAGE__){$p++for+1..55664
,;!print"Just another \u$p hacker,\n"}