Actually, what you want to do first is pull all the values out. Because, technically, there $a and $b are not the same and thus not common. It's your human perception that sees a list where perl is seeing a single value for each $a and $b.

So the first step is to split the strings into lists. Something like this:

  my @a = split /\s*,\s*/, $a;
  my @b = split /\s*,\s*/, $b;
[download]

The next step is to find the unique values. Let's say we did that and put it into a list named @c. To get back to the string, we use join:

  my $c = join ', ', @c;
[download]

Now all you're left with is the part in the middle. I'd like to leave you with something to do, so I'm going to just give you a hint: look up how to use hashes - they can help you keep track of what you've already seen.

Good luck!