die "FOO was not intended to run on OS $^O\n"
  unless defined($foo_path);

##</code><code>##

given ($^O) {
  when (/linux/)      { $file_path = '/mnt/foo' }
  when (/mswin32|nt/) { $file_path = '\\\C:\foo.exe' }
  default { die "FOO was not intended..." }
}