Porting/corelist-perldelta.pl: fix typo in error message
Porting/corelist-perldelta.pl: fix minor POD error
Porting/corelist-perldelta.pl: fix bug with 5.*.10 releases

The two aliases of the new Perl version (in this case, "5.01901" and
"5.019010") were being selected as the two versions to compare.
Unsurprisingly, those two versions have the same modules, so the
generated summary of changes was empty.