sub f {
@a = split //, shift; $ih = shift;
for $i (0 .. $#a){
        $e = $a[$i]; ${$ih}{$e} = 1;
        for $y ($i+1 .. $#a){
                $e .= $a[$y]; ${$ih}{$e} = 1;
        }
}
}

($l, $m) = split /\s/, <DATA>;
$_ = <DATA>; chomp; %h = (); f($_, \%h);

while(<DATA>){
        chomp; %th = (); f($_, \%th);
        $h{$_}++ foreach (keys %th);
}

foreach $key (keys %h){
        if($h{$key} == $m){
                $r[length($key)] = [] if ! exists $r[length($key)];
                push $r[length($key)], $key;
        }
}

print "@{$r[$#r]}\n";

__DATA__
3 2
ac
bc
b
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1: creates sub named f, which takes two parameters: string and a reference to a hash
that sub puts all combinations of substrings out of the given string into the hash
2: splits the first string from the file into two variables $l $m
3: takes the next line from the file and sends it into sub f with the reference to an empty hash %h
4: at this point the first line from the file is split on all its substrings which are put into
hash %h and have a value 1
5: then we read the rest of the file line by line and send these lines to the sub f along the reference
to an empty hash %th, right after that the two hashes are compared and the %h hash is incremented on the doubled values
6: runs through the %h hash and if the value of the key is amount of overlaps we need, then put it into an @r array of arrays
7: the last line prints all the longest overlaps with the same length