According to the 3rd Blue Camel, "." should be in the default @INC, so adding it shouldn't be necessary for 5.6 and before. However, my Perl 5.8.1 shows this:
in reply to use lib './' security safe?
hbo@owen|1347> perl -e 'print join "\n",@INC'
So it appears that the default has changed for 5.8, which came out after the last edition of Programming Perl.
Update: This is wrong. See beable's correction below.
Since use et al take the first match, there shouldn't be a danger of loading a bogus standard module if you have "."at the end of @INC. It's similar to having "./" at the end of your Unix PATH, however, in that what "./" means changes with your CWD. This opens up a possibility of loading a non-standard module that you don't expect. If you have root, (or administrator for *ix challenged) it's good practise to install your modules in the site_perl directories.
For my personal software, I create and use a ~/lib/perl and include the following at the top of my scripts:
That way I avoid the pesky relative path.
use lib /path/to/my/home/lib/perl;