#takes first line from the <DATA> and split values by ' ' into $lines 
+and $matches
($lines, $matches) = split /\s/, <DATA>;

#takes the next line from the <DATA>, chop off the \n and split result
+ed string
#into @a array by symbols
$_ = <DATA>; $_ = $` if /$/; @a = split //, $_;

#in this cycle(1) we create all possible combinations of substrings ou
+t of the 
#@a array, (out of the first line) and equals them to 1
for $i (0 .. $#a){
        $e = $a[$i]; $hash{$e} = 1;
        for $y ($i+1 .. $#a){
                $e .= $a[$y]; $hash{$e} = 1;
        }
}

#in this cycle(2) we read file line by line and for every line
#we do exactly the same as the previous cycle but into 
#temporal hash and then in the foreach cycle(3) we increment
#existed keys from the first hash if they are in the current line

while(<DATA>){
        $_ = $` if /$/; @a = split //, $_; %thash = ();

for $i (0 .. $#a){
        $e = $a[$i]; $thash{$e} = 1 if defined $hash{$e};
        for $y ($i+1 .. $#a){
                $e .= $a[$y]; $thash{$e} = 1 if defined $hash{$e};
        }
}
foreach $key (keys %hash){
        $hash{$key}++ if defined $thash{$key};
}

}

#and finally here we go through the hash
#and print only those keys which have their value == $matches 
$max = '';
foreach $key (keys %hash){
        if($hash{$key} == $matches){
                print "$key\n";
#               $max = $key if length($max) < length($key);
        }
}

print "$max\n";

__DATA__
3 2
strrringggg
ssttrrringggg
stttrrringgg
[download]

this whole script has a flaw:
the whole resulting hash is build upon the first text line
so in order to fix it in the cycle number 3 if the hash value is undefined you
should create one, not omit like in this example

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