Apologies for not being clear.

Apache opens the script and holds it open for its lifetime. RewriteMap writes to it (hence the while(<STDIN>){..} loop) and waits for output from STDOUT.

For every such mapping (read: every http request), the lookup method is called. I assumed that that is where the main speed up could be attained.

Although I'm not sure how moot this is with merlyn's reply in play