I'd suggest that you try implementing a couple of sample projects in Perl, and come up with estimates for how productive you are in Perl.

How productive Perl is per line depends on your coding style, knowledge of techniques, how well a problem fits Perl's strengths, whether there is an appropriate CPAN module, etc. It is extremely variable. But the variability will go down a lot if you measure one person, with the kinds of projects that that person tends to do. Furthermore if you're the person measured, then the estimate that you come up with is going to apply more directly to your project.