Well - consider how much time you will spend on clearing out the bugs in the legacy code, and consider how much time it will take you to produce tested, correct, efficient code from scratch. If you can reduce the 11,000 lines to 1,000 lines of lean, efficient, maintainable and correct code, it might be worth it - if writing those 1,000 lines takes roughly the same amount of time as fixing the flaws in the legacy code.

However, you could also argue that you should only rewrite those portion of the legacy code that is truly horrible. Is everything in the legacy code really so bad?

Anyhow - start by carefully considering how much time you'll need fixing the legacy code and how much time a re-implementation will take.

On a more general note, Perl can be very efficient, or very inefficient - both in terms of time to develop a solution and in terms of runtime speed. Usually, you need quite a bit of experience before you learn to write efficient, maintainable code in Perl!