I've never used the new version syntax (v1 = "\x01", v1.2 = "\x01\x02", etc.), but I finally got bitten by it. I was using LWP to for a post request in which some of the parameters were named v1, v2, etc., and I did this:
$req = POST "$site/execute_search.wmt",
tab => 'search',
# more parameters, including several like this:
v16 => $num_date,
So of course I was unknowingly sending a bunch of parameters with control-character names. Just when I'd gotten used to leaving off the quotes around things like 'time' and 'm' when they occur before the autoquoting =>, now I have to start worrying again! I guess it's really not a bug, though, just a 5.5-to-5.6 trap.
And of course there was enough other weirdness going on (cookies, nonstandard redirects, and other barriers to normal retrieval), that it took me a couple of hours to find the problem.