This is a CGI script?

mod_perl is good for high volume sites because it doesn't require the Perl interpreter to restart with each new request. However, there is some difference in how you write scripts that target mod_perl. In particular, variables should be lexically scoped. But that's not the only difference. You've got a little time before things get critical; get a good book on mod_perl and wade through it. You'll find that if you programmed your script thoughtfully the conversion may not be too difficult.

If there is database access, there is a module in Apache2 that can handle your database requests in a persistent process too. I'm not sure how good Perl's support is for this feature, but it's available.

Be sure to read the Apache documentation with respect to performance tuning. Likewise for database tuning if you're using database access.

And then profile your script. See if there are any bottlenecks to address.

Breaking it up into smaller chunks probably won't make a huge impact unless you're dealing with many thousands of lines of code. I would seek other bottlenecks first.