For someone who already has extensive programming experience I recommend picking up a copy of Programming Perl, reading it cover to cover (pay special attention to the Security chapter) writing out examples of concepts along the way. After that write a couple small practice projects and then he should be ready to get started.

Depending on how fast a reader he is, this may take a couple weeks, but it's important not to rush it too much or he could end up with some very poor quality (and dangerous) code.

Update: other nodes that may be of use:

• Not-So-FAQ about learning Perl
• Does Knowing Perl Help or Hinder Learning another Language
• How did you learn Perl?
• The best way to start learning Perl?

Another Update: As for sites to look at Ovid's CGI Course is good. I usually make a point of telling people not to go looking for other online tutorials because most of them encourage such bad practices.