If you don't know it already, perl 5.8.0 was released earlier this week. It now uses it's own IO subsystem (perlio) by default instead of the system's. I've already gleened one serious improvement from it... If you've been using String::IO or tied handles to create in-memory files, you should check out the ':scalar' layer.
open( $fh, "+<:scalar", \$buf );
My early benchmarks show it to be 10x faster than a simple buffer created with tie and 20x faster than String::IO! I'm testing out a copy of Apache::Filter I hacked up to replace the tied handle with the ':scalar' layer and the improvement is quite dramatic:)