It doesn't work because Perl is not C. Scalars in Perl are SVs (and Perl strings are PVs, not chars). Also Perl passes and returns values on a stack (not the C stack), not through C's calling conventions.

Read perlxstut to understand how passing and receiving parameters work. Then browse perlapi for specific functions. I believe (without reading it myself this morning) you need to check that you have a valid RV (something like SvRVOK) -- which represents a reference in Perl -- then access the referent and assign its PV (potentially upgrading it to contain a PV).