That should be taken from a Config.pm setting. I'd expect it to be in a CCFLAGS or something similar because expecting every single C compiler in the world to support -I just seems a bit optimistic so you should specify the whole command-line argument not just the path string.

I see $Config{incpath} but I expect that to be used for searching for *.h files from Perl code, not for constructing command-line flags to be passed to the compiler to tell it where to find include files.

Updates: Note that supporting $ENV{INCLUDE} or "... Makefile.PL INC=-I..." or other such things to augment CCFLAGS is just fine.

It sounds like you've found code that has some "-I" string hard coded and appends $Config{incpath} to that. To be clear, I don't consider such portable (in at least two ways).