The "cost" of creating a new instance of the Perl interpreter even in a "brute-force" CGI situation, is actually not as severe as you might suppose. Operating systems are designed to be "lazy" about loading and unloading program-segments, and file-data, from memory.

The "speed differences" that you might be concerned about are for the most part measured in microseconds, or at-most a few milliseconds. Which usually makes them "a mere trifle." Of far more importance is the fact that your web-application does its job efficiently and well. Choose good, sensible algorithms (leveraging CPAN anywhere you can), and you should be good-to-go.