I am pretty sure that Devel::Cover will not report modules that are within the @INC path.
Any values in that are in PERL5LIB will be also ignored by Devel::Cover. And I know in older versions, the value of PERL5LIB was taken at the time you compiled Devel::Cover, and was not updated if PERL5LIB changed.
The last possibility might have something to do with your line (my $ref = \&abcd::log;) and how Devel::Cover deals with such things.
If 1 or 2 are not the case, then I would suggest emailing the author (or the perl-qa list, where he has been known to hang out) before you submit a bug, since it might not actually be one (#3).