I think that perldoc perlre as a reference and perldoc perlretut as a tutorial for regular expressions work well. If you want a book that goes far beyond this, have a look at Mastering Regular Expressions, although I don't know how good it is at introducing regular expressions.

Some pointers to get you started on your homework:

1. A match can be naively negated in two ways:

   if ($item !~ /foo/) {};
   if (! $item =~ /foo/) {};
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   Experiment with these to find the solution for the first exercise.
2. For removing the first char of a string, perlfunc -f substr might be of interest, but as you want to limit yourself to regular expressions, the substitution operator s/// will be of interest to you. perldoc -f split might also be a way to a solution, but you will also need perldoc -f join to complete that way.
3. This is a hard problem to solve with regular expressions for someone new to REs. I would use one of the prefabricated modules for parsing CSV strings, but if you really want to go down the road of parsing CSV with regular expressions, approach the problem in the following fashion:
   1. read about negated character classes in perlre
   2. find a RE that matches a quoted string. A quoted string is a string that starts with a double quote, ends with a double quote and contains no double quote between those two.
   3. find a RE that matches a comma within a quoted string
   4. find out how to combine smaller REs into larger REs

Personally, I wouldn't approach many of these problems with regular expressions, as the time invested to craft such a special RE takes much too long for me, when a step-by-step solution is immediately available.

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
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