Use defined:

die "FOO was not intended to run on OS $^O\n"
  unless defined($foo_path);
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Update: In 5.10 you can write:

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

You might even be able to factor the assignment to $file_path to outside the given statement -- gotta get 5.10 installed and try it out.