Daniel R. Allen did a presentation on Style Guides to the Toronto Perl Mongers last week, and since I had been meaning for a while to research the topic, I gathered together links to relevant information. Dan's research for his talk included this Perlmonks discussion thread
While I have preferred capitalizing words, the documents I found convinced me to change to using underscores. The spacing makes it easier to see where one word ends and another begins. While the underscores make the word longer, that discourages excessively verbose variable names.