The problem is, not too many tools are available at that point during the build.

What I've determined so far is:

• The problem only occurs when running exetype.pl with miniperl.exe.
• Quoting the variables $offset & $size that contain unpacked numeric values entirely fixes the problem:

  # <perl>/win32/bin/exetype.pl
  ...
  37: seek EXE, "$offset", 0;
  ...
  43:die "Optional header is neither in NT32 nor in NT64 format"
  44:    unless ("$size" == 224 && $magic == 0x10b) || # ...
  45:           ("$size" == 240 && $magic == 0x20b);   # ...
  [download]

  Nmake test is still running, but is sufficiently far through that I don;t expect (extra) failures now.

• Using 0+$offset & 0+$size doesn't effect a cure.

My speculation is that unpack in miniperl is setting SvIOK when it shouldn't be.