I'm afraid that that doesn't make any difference; neither does using \xD2 instead. There's a section in the perlre documentation which implies \322 only means a previously captured match if I've got 322 brackets (although I'm possibly misreading it).
Further, using  $a=chr(210); s/.*?$a/yes/; doesn't work either.
However, all the problems go away if I use \177 (ie. 127 decimal) or lower. So there's a definate inconsistency. :-(