while( $u = <STDIN> ) {
    for( $i=1; $i < length($u)-1; $i++ ) {
        for ($j = 0; $j < length($u)-$i ; $j++ ) {
        $s = substr($u, $j, $i)
        $matches($s) += ( $u =~ m/$s/g );
    }
}

# now loop over %matches finding top 30 of:
#     $matches($key) * length($key)
#