It's a combination of several things. Here's the biggies.

• Using mod_perl
• Implementing your program(s) as an apache handler or Apache::Registry
• Pre-loading all modules
• Caching all needed data and expensive queries (this really helps)
• Optimal SQL queries if you're using some type of database
• General coding efficiency (writing tight code, using references, optimising accessor functions, etc.)
• If you're returning huge pages, mod_gzip really helps cut down on transfer time
• Tuning apache's MinSpareServers, MaxSpareServers, etc
• Use tt2 instead of HTML::Template. It's great, and the new XS-based Template::Stash is spiffy

Hope this helps.