So, incorporating abell and cfreak's suggestions, you get:

#!/usr/bin/perl -w

@screen = ( ' 'x70 . "\n" ) x 20;
$gen_line = sub {
    my $qw;
    for(1..70){
        if( !$q[$_] && rand > .993 ){
            $q[$_] = q$.$ x 5 . q$ $ x 1 
                . ${ [ qw$tsuJ rehtona lreP rekcaH$ ] }[$qq++]
                . q$ $ x 1 . q$.$ x 5;
            $qq = $qq > 3 ? 0: $qq;
        }
        if( $q[$_] ){
            ( $qx, $q[$_] ) = split//, $q[$_],2;
            $qw .= $qx;
        }else{
            $qw .= q$ $;
        }
    }
    "\33[0;32m$qw\33[0m";
};

while(1) {
    print join '', @screen;
    # Drop last line
    pop @screen;
    # Put a new line on top
    unshift @screen, $gen_line->() . "\n";
    # Sleep for 0.1 seconds
    select(undef, undef, undef, 0.1);
}
[download]

Cool, monks! =]