Imploding URLs

[mobiGeek]

I want to "implode" URLs, that is shorten them by replacing long and/or frequently seen substrings with single non-visible ASCII chars. Essentially, the code I have is:

my $tnt = [
           "http://www."    ,
           "https://www."   ,
          };

my $implode_url = sub {
    my ($url) = @_;
    my $len = scalar(@$tnt);
    for( my $i = 0 ; $i < $len; $i++) {
        $url =~ s/$tnt->[$i]/chr($i+1)/eg;  # +1 to avoid \0
    }
    return $url;
};

What I'm looking to do is, given a large list of URLs find the top 31 (chr(1) to chr(31)) most valueable substrings to populate $tnt.

Obviously, the most valuable is a measure of the substring's length x frequency within the collection.

So, given a collection of URLs (one per line), does anyone have a suggested algorithm outside of brute force (pseudo-code below)?

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)
#

Thanks in advance!

mG.