It's possible (I guess) that you have an outdated version of one of the modules here that would somehow require B::Bytecode. (The first thing I'd try is updating them, anyway.) One way to see where the problem is is to precede every use with a diagnostic message:

BEGIN { warn "About to use a"; }
use a;
BEGIN { warn "About to use b"; }
use b;
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(The BEGINs are necessary because use statements are implicitly wrapped in BEGINs themselves.) If you don't see About to use b, then you'll know that the error was caused somewhere in the course of executing use a.

UPDATE: On further reading (and a bit of clumsy line-counting), it looks like the error is being thrown on the DBI->connect line. I don't have DBI installed, so can't test; but maybe someone else knows whether this method might try to use B::Bytecode for some reason?