Thanks: idling away some slack time if taxis returns too quickly seems to do the trick. I knew the sim would take longer with larger numbers of lice, the problem was mostly that with a small population and the simple while loop, the lice looked like they were on amphetamines! The only change I have made is to use...

DoOneEvent( DONT_WAIT ) until Time::HiRes::time-$start > MIN_REFRESH;
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Since DoOneEvent() alone frequently takes longer than 20 ms to return, and therefore the sim gets very jerky with small populations.

Thanks again!