Okay, I guess I should have looked a little further before posting... here's a relevant snippet from the "perldelta" man page that comes with 5.8.8 (though it's a little bit less than fully reassuring, and doesn't actually mention the specific problem I reported above):

   Debugger and Unicode slowdown

   It had been reported that running under perl's debugger when proces
+sing
   Unicode data could cause unexpectedly large slowdowns. The most lik
+ely
   cause of this was identified and fixed by Nicholas Clark.
[download]

Well, on to getting 5.8.8 installed on my mac... Sorry about the noise.