perl-module: Fix return value of append_case subroutine.

The casewriter_write() function does not have a return value (it is
declared as "void"), but the append_case subroutine tried to use its
return value anyway.  This cannot work properly, of course, and in fact
caused problems on 64-bit builds in particular.

I don't know why there is no C compiler warning about this problem.  I
guess the Perl module build must somehow turn off a lot of GCC warnings.

Reported by bojo42 <bojo42@gmail.com> in bug #31611.