Two to one was pretty much a guess, but based on the sort of C++ code I'm involved in which tends to be interacting with an instrument over USB and data analysis. I'd guess in those areas Perl wouldn't have so much advantage.
On the other hand I have a modest size C++ app (maybe 2000 lines) that I'm slowly rewriting in Perl. In that case I expect to get about a five to one advantage, but it is processing email and I can use MIME::Lite to replace a swag of C++. That particular application is made for Perl and I regret not knowing Perl when I chose C++ to write the first version!
DWIM is Perl's answer to Gödel