I have many of the O'Reilly books + "Effective Perl Programming" and
they're all great. But once you I got beyond a certain
level I found a few techniques for improving my perl skills that seem
to work well:
Reading Perlmonks... I sometimes use
Seekers of Perl Wisdom
to test and broaden my skills.
When someone asks a question that I don't know the
answer to I will sometimes
dig-in and see if I can figure it out before reading other's
answers.
Browse through CPAN to just learn what's new out there.
Discover new modules and figure out how to use them.