Hey

I misread the definition of substring to mean all substrings of that length or lower...quick easy fix and it starts placing decent.

sub common_sub {
    my ($s1,$s2,$len) = @_;

    my $len_s1 = length($s1);
    my $len_s2 = length($s2);

    my $match_s1 = {};
    my $match_s2 = {};
    
    for my $start (0..length($s1)-1) {
        #for my $l (1..$len) {
            next if $start+$len > $len_s1;
            $match_s1->{substr($s1, $start, $len)} ||= 1;
        #}
    }

    for my $start (0..length($s2)-1) {
        #for my $l (1..$len) {
            next if $start+$len > $len_s2;
            $match_s2->{substr($s2, $start, $len)} ||= 1;
        #}
    }
    $match_s1->{$_}++ for keys %$match_s2;
    return grep { $match_s1->{$_} == 2 } keys %$match_s1;
}
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