Why were you asked to rewrite in C++? Was any justification given? Or is it just a blind assumption that it will be better?
I am going to assume that the perl script is a complicated beast.
I would seriously advise
against a rewrite, except as a last resort. Complex code that works well is usually the result of a lot of testing and debugging. I would not cast it aside lightly.
I concur with Corona688 and elixir_sinari. Profile your current script and see where it spends most of its time. Then, have a look at those subroutines and see if they can be optimized in perl. Even if that's insufficient, a complete rewrite is still not your only alternative. The critical perl sections can be re-implemented in C and loaded by the perl interpreter like any other module:
perlxs.
One thing is certain: advice on whether to rewrite code from people who do not know what that code does isn't worth much. Perhaps not even the proverbial 2 cents.
Regards,
Alister