Not minding that Perl just isn't suited to bit-fiddling tasks like this, I wanted to see the CPU-meter of my new machine go to 100%. I implemented a threaded version of your program, which runs about 3 to 4 times as fast on my 4-CPU machine by processing sets of 10000 keys in every thread.

The conversion to threaded code was fairly easy by using a queue through which all batches of keys to be tested go through. The main thread produces batches of 10000 keys and stuffs them into the queue, and goes to sleep once it finds that the queue contains more than 4 batches. Each worker thread retrieves a batch and crunches on it.

In a real program, there would be a second queue for communicating the status/success of every thread back to the main thread.

#!/perl/bin/perl
use strict;
use Crypt::RC4;
use Time::HiRes;
use threads;
use Thread::Queue;

my $max_threads = 4;

my $data1="C5C444220AD6FB08972792318300CC703814513EE013AF97B94FF9ACF23
+F9C8F0E748C04B2E18BB5A1B491BB73E23EC9A4233B9BCDE4854FD03DCBAC4B3E9EA0
+0F5BF7A3A119B0FF2E66C9DD96E7F4F0972959082601AA5DD202DFB0
5039CA4FDC280517E244353690C0DE1A";

my $foo1=pack('H*',$data1);


my $dt=time;
print "$dt Start \n";

my $batchsize = 10000;
my $key="A";

# The message queue through w
my $jobs = Thread::Queue->new();

# Create the worker threads
for (1..$max_threads) {
    async {
        my $ct;
        while ( 1 ){
            my $start = time;
            my $items = $jobs->dequeue;
            my $SKIP=0;
            for my $key (@$items) {
                my $ucrypt=RC4($key,$foo1);
                my $clen = length($ucrypt);
                my @uchar = split(//,$ucrypt);
                for my $i (0..$#uchar) {
                   my $val = ord($uchar[$i]);
                   if($val>127) {
                       $SKIP=1;
                       last;
                   }
                }
                if ($SKIP == 0 ){
                    print "matched using key:$key \n";
                    print "$ucrypt \n";
                }
            };
   
            my $end = time;
            print $end - $start," seconds\n";
            printf "%0.2f items per second\n", @$items / ($end - $star
+t);
        }
    };
}

while (1) {
    my @batch = map { $key++ } 1 .. $batchsize;
    $jobs->enqueue(\@batch);
    my $sleep = 1;
    while ($jobs->pending() > $max_threads) {
        sleep $sleep;
        $sleep *= 2;
    };
}
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