Surprisingly, no one managed to hit the reason why this fails.

Sorry for tooting my own horn here, but some of us did :-) .

In your original code, $filehandle is referencing a global glob. With your new code, $filehandle is now ... undef.

Therefore, give the overriding open a non-undef lexical handle instead: open(my $AFILE = IO::File->new(), ... ). This solution has the virtue that it does not require that the overriding open be modified, something that may not be possible or desirable.