$ perl 939870.pl 1000
Building arrays of 1000 with unique 4-char keys... done.
Arrays have 1000 items each. Benchmarking...
                    Rate  arrays plus regex     using a hash
arrays plus regex   2.70/s             --            -100%
using a hash    1000000000000000/s 37000000000000000%  --              

$ perl 939870.pl 5000
Building arrays of 5000 with unique 4-char keys... done.
Arrays have 5000 items each. Benchmarking...
                    s/iter arrays plus regex      using a hash
arrays plus regex     9.40                --             -100%
using a hash      1.00e-02            93900%                --

$ perl 939870.pl 10000
Building arrays of 10000 with unique 4-char keys... done.
Arrays have 10000 items each. Benchmarking...
                    s/iter arrays plus regex      using a hash
arrays plus regex     46.3                --             -100%
using a hash      3.00e-02           154267%                --

$ perl 939870.pl 100000
Building arrays of 100000 with unique 4-char keys... done.
Arrays have 100000 items each. Benchmarking...
                  s/iter arrays plus regex      using a hash
arrays plus regex   4110                --             -100%
using a hash       0.330          1245345%                --

##</code><code>##

$ cat 939870.pl
#!/usr/bin/perl
use Modern::Perl;
use Benchmark qw(:all);

# create two lists, with the formats:
# list1: (four characters A-Z)
#   RDAE
#   JFCS
#
# list2: (7 digit number, space, four chars A-Z)
#   1234567 JIQZ
#   2345678 EFOP

my( @list1, @list2);
my( %keys1, %keys2);
my $array_size = $ARGV[0] || 100_000;

print "Building arrays of $array_size with unique 4-char keys... ";
for (1..$array_size){
    my($s1, $s2) = ('','');
    $s1 .= ('A'..'Z')[rand 26] for (1..4);
    $s2 .= ('A'..'Z')[rand 26] for (1..4);
    next if $keys1{$s1} or $keys2{$s2};    # make keys unique
    push @list1, $s1;                      # save keys in lists
    push @list2, int(rand(8999999)+1_000_000). " $s2";
}

say "done.\nArrays have ". 
    scalar(@list1).
    " items each. Benchmarking...";

cmpthese(1, {
    'arrays plus regex' => \&array_plus_regex,
    'using a hash'      => \&using_hash,
});

sub array_plus_regex {
    my @results;
  FOUND:
    for my $l1 (@list1){
        for my $l2 (@list2){
            if($l2 =~ / $l1/){
                push @results, "$l1 ---> $`";
                next FOUND;
            }
        }
        push @results, "$l1 ---> null";
    }
}

sub using_hash {
    my @results;
    my %match;
    my $count = 0;
    for my $l2 (@list2){
        my($k, $v) = split ' ', $l2;
        $match{$k} = $v;
    }
    for my $l1 (@list1){
        if($match{$l1}){
            push @results, "$l1 ---> $match{$l1}";
        } else {
            push @results, "$l1 ---> null";
        }
    }
}