You are probably using some case-insensitive file-system. (update: Using "b" is dangerous for case-insensitive filesytems) Don't use the B package names for yourself, B a core module.

edited: something else is going on, not yet sure what, though.

edited: apparantly, this is some kind of nasty interaction with compiletime/runtime: this error only occurs if you load a.pm as a normal code (instead of useing a.pm):

test.pl:

#!perl
use lib '.';
use a;
[download]

end then:

> perl -c test.pl
[ ... lots of output omitted ]
test.pl syntax OK
[download]

I don't have the time to investigate this further, though.