The sources are written in CWEB. The header file generated from the CWEB input is incomplete, and the XS module uses implicit function declarations. I spot-checked and these functions return int, so this hopefully is okay.

Given that this Perl module does not seem to see active development, it doesn't look like it's worth porting it to C99.  Setting the compiler to CC="gcc -std=gnu89" is properly honored, so implementing the exception is easy.