If this goes ahead, please do not make it look for nmake.exe in some particular path. Rather, just try invoking it via the shell (eg.

my $banner = `nmake /?`;
[download]

The freely distributable version of nmake (v1.5) is useless for building many applications that require later versions, and having the installed later version blindly and silently overwritten or preempted by an unnecessary "clever hack" is intensely frustrating.

Personally, I think it would be much better to simply output a clear message if it cannot be found:

nmake.exe not found. Either make this available via your path variable, or download it from ...