my %hash;
@hash{@l2} = @l2;
my @newlist = @hash{@l1};
[download]

Update: Ah, after two downvotes, I see that the OP wants the string 'null' rather than actual null values. Well, that certainly is worth throwing dung at someone over. Sheesh. One change to the final line:

my @newlist = map defined($_)?$_:'null', @hash{@l1};
[download]