General trouble-shooting tips:

1. Sanity dictates that if "nothing changed", and something broke, then something was changed.
2. Occam uses his Razor more often than not.
3. For the sake of argument, assume the code is ok (but see rule 1). So that means something is wrong with the environment. Check that first, for example, check in your browser that the cookie really is being set.
4. If the environment really looks ok, check it again.
5. If it still looks ok, have someone else check it.
6. If it still looks ok, are you sure there is no loose nut on the keyboard, and that PEBKAC does not apply? Also check for ID-ten-T errors.
7. If you still can't find a problem (and still bearing in mind rule 1), check the code. Use general debugging techniques to narrow down the problem. Run your unit tests and verify they still complete successfully.
8. Avoid absolute statements like "the problem must be with the frobnitz confrogulator". These blind you to avenues of investigation you might otherwise explore.

Given the nature of your problem, and if you are really sure the code was not changed, most likely something with the browser or the user is not what you expect. You would need to post more of your code, including the login.pl script, for anyone here to be able to debug it.

Update: added absolute statements tip.