I don't want to bash String::LCSS, but the implementation seems to be the naive O(n^3) algorithm instead of the O(mn) dynamic programming solution (http://en.wikipedia.org/wiki/Longest_common_substring_problem). A quick and dirty (and not thoroughly tested) implementation is much faster (although probably buggy).

sub lcss2 {
    my ($s, $t) = @_; 
    my $z = 0;
    my $m = length $s; 
    my $n = length $t; 
    my @S = (undef, split(//, $s));
    my @T = (undef, split(//, $t));
    my @L; 
    my @ret;
 
    for my $i ( 1 .. $m ) { 
        for my $j ( 1 .. $n ) { 
            if ($S[$i] eq $T[$j]) {
                $L[$i-1][$j-1] ||= 0;  
                $L[$i][$j] = $L[$i-1][$j-1] + 1;
                if ($L[$i][$j] > $z) {
                    $z = $L[$i][$j];
                    @ret = (); 
                }
                if ($L[$i][$j] == $z) {
                    push @ret,substr($s, ($i-$z), $z);
                }
            }
        }
    }   
#    warn Dumper \@L;
    return join '*', @ret;
}
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my $s1 = '6'x 200 . 'zyzxx';
my $s2 = '5'x 200 . 'abczyzefg';
my $count = 1;

timethese($count, {
    'String::LCSS' => sub { String::LCSS::lcss( $s1, $s2 ) },
    'dynprog' => sub { lcss2( $s1, $s2 )},
});
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