I strongly smell client-side (IE) issues here, not server, and definitely not Perl. (There is a chance that the server plays into it, such as by not faithfully rendering your expiration or nocache pragmas).

I have two debugging suggestions:

1. Can you put a changing token (such as e.g. a time() output) into a hidden form field and see if that changes things? When dealing with hard-to-uncover oddities such as this, I often find it is helpful to ensure that (at least second-by-second with the time() token) each transaction is indeed unique. This helps to cut down on cache-related fubars.
2. Consider using javascript to capture onClick and onSumbit events to see what is happening. Simply doing onClick="alert('clicked')" or onSubmit="alert('submitted')" in the button and form, respectively, might clarify things and lead you on the path to more sanity.

Good luck!