You could also try moving #include "stopwatch.h" before the perl xs includes, worked for me before (see Re: perlxs and c++ : dosish.h(111) : warning C4005: 'Stat' : macro redefinition),
but thats a shot in the dark.
If you want more help you should try posting stopwatch.h along with other files needed to try to compile this module.
MJDsays "you can't just make shit up and expect the computer to know what you mean, retardo!"