That's what hard is supposed to be for (not adding interval after, but before). Make sure you include Time::HiRes. From the manpage:

If the "hard" flag is set, the event will be
queued for execution relative to the last time the
callback was invoked.  However, if "hard" is false the
new timeout will be calculated relative to the current
time (this is the default).
[download]

If you set repeat and hard, it shouldn't creep. I start up my samplers parked, set the at and interval, and then start them.

How fast are you getting callbacks? If you are taking too long in some of the callbacks, you could bump the callback time. Make sure that your callback action can never overlap the next callback by either speeding up the callback or slowing down the rate. Also, you might try setting debugging on either globally or on the watcher; there are diagnostics printed as to the actual times between callbacks, etc.