1. You don't need a loop to do character matching, and
2. You don't need to call an external program to get the current date.

Try something like this:

#!/perl/bin/perl
use warnings;
use strict;
use Crypt::RC4;

my $foo1 = pack 'H*', 'C5C444220AD6FB08972792318300CC703814513EE013AF9
+7B94FF9ACF23F9C8F0E748C04B2E18BB5A1B491BB73E23EC9A4233B9BCDE4854FD03D
+CBAC4B3E9EA00F5BF7A3A119B0FF2E66C9DD96E7F4F0972959082601AA5DD202DFB05
+039CA4FDC280517E244353690C0DE1A';

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

my $key = 'A';
my $ct;
while ( 1 ) {
    my $ucrypt = RC4( $key, $foo1 );

    if ( ++$ct == 10_000 ) {
        $dt = localtime;
        print "$dt count $ct key: $key\n";
        $ct = 0;
        }

    if ( $ucrypt !~ /[\x80-\xFF]/ ) {
        print "matched using key:$key\n$ucrypt\n";
        }

    $key++;
    }
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