It is worth mentioning that most Windows programs (nearly all, with the notable exception of CMD.EXE and Explorer.exe) do understand forward slashes as "path separator".

So, for most system functions outside of Batch "Scripts", you can use "C:/Windows/".

If in doubt, use e.g. catfile/catdir of File::Spec or canonpath of Path::Tiny.

Update: in Stackexchange's "Retrocomputing" section someone writes the following, which coincides with my experience:

All dedicated path names, like in syscalls, can be written with either slash. It's only within the command line scan of each command, that simple slashes get interpreted as switch indicators.