my @l1=("Date","IndexID","Maturity","OnTheRun","CompositePrice","CompositeSpread","ModelPrice","ModelSpread","Depth","Heat");
my @l2=("OnTheRun","CompositePrice","CompositeSpread","Depth");

##</code><code>##

foreach my $f (@l1){
        if (join(",",@l2) !~ /\b$f\b/){
          $f='null';
        }
      }

##</code><code>##

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

?