You might try to clarify with your boss, is he expecting a wild estimate, an educated guess, or something more definite? Personally, I hate these kind of questions.

philcrow's answer is a good one ... it reminds your boss of the cumulative savings over time of the reduction you made last year and avoids his 'what can you do for us THIS year' question.

In all fairness, though, I would challenge your statement: "... because such savings are impossible to do." I would not be surprised if further savings opportunities occurred to you to follow on from your success last year. If you came up with a creative solution last year, why not expect another such solution this year? Perl is a powerful language applicable to a large problem space ... don't sell yourself short.

Personally I would refuse to make a guess, because I'm uncomfortable drifting too far from ones and zeroes. At the same time, though, I would try to convince my boss to make some conservative assumptions based on last year's performance.