Thanks for all of your help!

I tried all of this out, initially testing it on 100 files of about 1MB each - inefficient I know, but it's the format my data originally came in (each file is a book). As you forsaw, threading for zillions of small files wasn't great, and your script actually increased my running time from 11 seconds to 17 seconds. However, when I took out threading but left the other stuff, it cut my running time in half to 5.5 seconds! Wonderful! I had grown accustomed to declaring variables separately more often than creating them on the fly, because I find it easier to read and maintain my code that way, and I never quite noticed how inefficient (at least in Perl) that is.

When I combined my input files from 100 1MB files to 4 25MB files, running the improved script without threading was reduced to 4 seconds, and putting threading back in only slowed it down a little: to 4.7 seconds - so I guess opening up the file takes a little longer that spell checking 25MB - or there is overhead for threading, or both. When I process the whole corpus I was planning on combining everything into about 1000 1GB files, and then I presume that the threading would only make things quicker. (Though it leaves me with a lingering, generic question that I was considering asking on Stack Overflow or somewhere cause it's not specifically Perl related: why not just have a single 1TB file if I'm only ever processing the whole thing? Not that a 1000 file opens takes up a large portion of an operation that takes several days, assuming that the time to open a filehandle is constant relative to the file size.)

Anyway, thanks so much for your help - you cut my execution time in half and pointed me in the right direction for future savings. Cheers!