Cygwin optionally supports case sensitive file names on NTFS partitions if a registry key is set. If enabled, Win32::GUI fails to build with Cygwin perl due to the following inconsistencies:
Custom typemaps are named 'TYPEMAP' or 'Typemap' instead of the 'typemap' expected by perl (see perldoc xsubpp)
In several places, #include "Windows.h" or "Winver.h" is written with a capital W, where the actual headers are installed lowercase.
In Win32-GUI_Scintilla/Scintilla.xs, #include "./include/Scintilla.h" is used while the actual directory is Include (with a capital I).
fixed in Win32-GUI-1.13