It works fine for me. In what way did it not work for you? I did not show the use strict; and use warnings; I always put at the top of my scripts in my first post. Here's the whole script again, it's output and some info on my platform and perl version

$ cat spw582186 
#!/usr/bin/perl
#
use strict;
use warnings;

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

my @l2 = qw(OnTheRun CompositePrice CompositeSpread Depth);

my @l3;

{
    local $" = q{|};
    @l3 = map {m{^@l2$} ? $_ : q{null}} @l1;
}

print qq{$_\n} for @l3;
$ ./spw582186
null
null
null
OnTheRun
CompositePrice
CompositeSpread
null
null
Depth
null
$ uname -a
SunOS b4rsk 5.10 Generic_118833-03 sun4u sparc SUNW,Ultra-60
$ perl -v

This is perl, v5.8.4 built for sun4-solaris-64int
(with 28 registered patches, see perl -V for more detail)

Copyright 1987-2004, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5 source ki
+t.

Complete documentation for Perl, including FAQ lists, should be found 
+on
this system using `man perl' or `perldoc perl'.  If you have access to
+ the
Internet, point your browser at http://www.perl.com/, the Perl Home Pa
+ge.

$
[download]

You may well be right regarding efficiency, I haven't run any benchmarks. The object of the post was to show an alternative method to those already given.

Cheers,

JohnGG