I tried using \xE4.
I tried byte 0xE4 (iso-latin-1).
I tried bytes 0xC3 0xA4 (utf-8) with "use utf8;".
I tried bytes 0xC3 0xA4 (utf-8) without "use utf8;" (shouldn't work).
I tried utf8::upgrade($date_in)
I tried utf8::downgrade($date_in).

None worked.

I'd contact the author. It's obvious that he intends one of those to work.

Update: Added last two, but they are redundant with earlier tests in this case.