Encode::FB_CROAK leaks memory in old versions of Encode:
<https://rt.cpan.org/Public/Bug/Display.html?id=139622>
Since I expect there's still many users on old systems and old
Perls, we can use "$SIG{__WARN__} = \&croak" here with
Encode::FB_WARN to emulate Encode::FB_CROAK behavior.