Thanks, all, for the very useful responses (I'm responding to BrowserUk's reply specifically because it's so detailed and helpful in so many aspects) - this was very useful for both confirmation and more direction in deciding where to look for the error. I'm not all that familiar with the guts of Perl, but it's looking likely that there's a major compilation difference responsible here: either single-precision lookup tables, or perhaps a radically different math lib. Annoying that something like that could affect a Perl program... but on the other hand, good to learn that it can.

Again, thank you all very much.