When someone loads your module via use or require, there is no gap between the finish of parsing your module and the execution of code in your module. Therefore there is no possibility of a race if you don't play games with BEGIN in your module, and stupid games are not played while checking who is loading you. I assume, of course, that you are nt using an unstable experimental feature. (ie Threads. And if Perl 6 gets into races with initialization code while loading modules, then that is a showstopper in my books!)
If I am wrong then please show me how exactly this race can happen in Perl. If it is something which I think could possibly happen to me, then I will start blocking it. But not unless. In general I won't put energy into defensively coding against things that I don't think I will get wrong. Conversely if it is something that I can conceivably get wrong by accident, I will become a paranoid nut. :-)
Now I will give you a very good reason not to move your initialization into BEGIN. If you don't move your manipulation of @ISA into BEGIN, then on 5.005 (and if they fix the stupid $Carp::CarpLevel games in Exporter on 5.6 as well) if you mess up a use statement then you will by default get it correctly reported from the correct package in your module rather than from the module which uses you. If you move the manipulation of @ISA into BEGIN or use base to achieve the same effect you will mess that up. (Note that the fixed Carp that will appear in 5.8 does not have this issue.) Therefore by not playing games with what parts of your initializations occur before your module is done compiling, you will get error reporting which is more likely to be informative.
So the BEGIN time initialization not only doesn't buy me anything that I care about, it loses me something that I consider very important!
The qr// point is a matter of taste and environment. No, it is not supported in older Perl's. If that is an issue for you, then it is easy enough to drop it and just split on /$re/.