Greetings, esteemed monks!

I have two lists, one of which is a subset of the other. So for example,

my @l1=("Date","IndexID","Maturity","OnTheRun","CompositePrice","Compo
+siteSpread","ModelPrice","ModelSpread","Depth","Heat");
my @l2=("OnTheRun","CompositePrice","CompositeSpread","Depth");
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I want to end up with a list that is the same length as @l1, but every value that is not also present in @l2 should be replaced with "null".
What I have so far:

foreach my $f (@l1){
        if (join(",",@l2) !~ /\b$f\b/){
          $f='null';
        }
      }
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I'm thinking there has to be a more elegant and/or efficient and/or more robust way.
perhaps something like

my @l3= map {(join(",",@l2) =~ /\b$f\b/)?$f:"null"} @l1;

?
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I guess what I don't like is the implied nested looping (I assume the join has to be re-done for each iteration through @l1?). Plus, the join just seems kind of ugly. Maybe it's unavoidable.

For the record, this is for Text::CSV::Simple and I am trying to make a field map.

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I like computer programming because it's like Legos for the mind.