I'm not entirely sure if I understood the first parameter correctly.
It's just a (redundant) check? No matter. 
chomp (my @dta = <DATA>);
my ($ntot, $nmin) = split /\s/, shift @dta;
die if $nmin < 2 or $nmin > $ntot or $ntot != @dta;

sub cpfx { ($_[0]^$_[1]) =~ m/^\0*/s; substr($_[0], 0, length $&) }
sub sufs { map {[substr($_[0], -$_) => $_[1]]} 1..length($_[0]) }

my @SA = sort {$a->[0] cmp $b->[0]} map {sufs($dta[$_], $_)} 0..$#dta;
my @N = (0) x @dta;
my ($best, $n, $h, $t) = ("", -$nmin, 0, 0);

while ($h < @SA) {
  ($n += !$N[$SA[$h++]->[1]]++) >= 0 or next;
  ($n -= !--$N[$SA[$t++]->[1]]) while $n > 0 || $N[$SA[$t]->[1]] > 1;
  my $pfx = cpfx(map $SA[$_]->[0], $t, $h-1);
  $best = $pfx if length($best) < length($pfx);
}
print "\"$best\"\n";

__DATA__
6 3
You can use the substr() function as an lvalue, in which case EXPR mus
+t itself be an lvalue.  If you assign something shorter than LENGTH,
the string will shrink, and if you assign something longer than LENGTH
+, the string will grow to accommodate it.  To keep the string the
same length, you may need to pad or chop your value using "sprintf".
If OFFSET and LENGTH specify a substring that is partly outside the st
+ring, only the part within the string is returned.  If the substring
is beyond either end of the string, substr() returns the undefined val
+ue and produces a warning.  When used as an lvalue, specifying a
substring that is entirely outside the string raises an exception.  He
+re's an example showing the behavior for boundary cases:

[download]

In reply to Re: longest common substring (with needed tweaks) by oiskuu
in thread longest common substring (with needed tweaks) by R56

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