Short answer

Instead of using POP use email forwarding from your other account to be able to use Filters and Canned Responses in Gmail

Explaination

it can't seem to do auto-replies

I beg to differ.

Make sure Canned Responses is activated:

Then you create a filter for emails that are sent to your old email address:

to:(my.old.address@example.com)

EDIT: Note that this will not work for mails fetched through POP3, only for forwarded messages. If your old account has an auto-forwarding feature, that's the way to go. (The other answer here pointed this out.)

You call the filter, say, Old. Then you select a canned response for that filter. Actually, you don't even need to do that. You can autorespond to these emails just the same just by creating a filter for them.

Look at this screenshot:

The thing to remember about the current "canned response" for Gmail is that it has to be added from the "Labs" tab.

This means that there may be sparse support and has potential to completely disappear. This particular "Lab" has a number of other quirks that are outside the scope of your question but I believe your answer is "No".

It appears that the reason for the "+canned.response" is to prevent some email loop that someone might inadvertently create by having the message resent to themselves or even just replied to.

In my test I used a filter for the response based on a specific "From" with a "Subject" containing a keyword. The canned response fired every time except when I replied directly to the canned response.

I'm sure that most people can live with this quirk, but some of the other quirks I came across in testing this out might cause Google to pull this Lab.