var path = "perl";
var found = Environment.GetEnvironmentVariable("PATH").Split(';')
  .Select(x => Path.Combine(x, path))
  .SelectMany(_ => Environment.GetEnvironmentVariable("PATHEXT").Split(';')
    .Concat(new String[] { Path.GetExtension(path) }),
      (p, ext) => Path.ChangeExtension(p, ext))
  .Where(File.Exists).FirstOrDefault();
// found => "c:\cygwin\bin\perl.EXE"(インストール済みの場合)