Not so efficient, but output divided by words and counted. Sorting substrings alone is easy.

use strict;
use warnings;

my $words = 1;   # Phrase must contain at least 1 word
my $size = 5;    # Phrase must be at least 5 characters long
my $matches = 2; # Must be at least 2 copies of phrase

my (@words, @pos, @matches, $p, $c, $key);

$_ = join '', <DATA>;
$_ = lc($_);
@words = m/\w+(?:'\w+)?/g;
@pos = sort { mycmp($a, $b) } 0..$#words;

for $p ($words..$#pos) {
    $c = mycount($p);
    for (1..$c) {
        $key = join ' ', @words[$pos[$p]..($pos[$p]+$_-1)];
        next if length($key) < $size;
        $matches[$_]{$key}++;
    }
}

for (reverse $words..$#matches) {
    print "$_ words:\n";
    $c = $matches[$_];
    for (sort { $c->{$b} <=> $c->{$a} } keys %$c) {
        last if $c->{$_} < $matches - 1;
        print "    $_ : ",($c->{$_}+1),"\n";
    }
}

sub mycount {
    my $x = $pos[$_[0]];
    my $y = $pos[$_[0]-1];
    my $c = 0;
    $c++ while $x <= $#words && $y <= $#words && $words[$x++] eq $word
+s[$y++];
    return $c;
}

sub mycmp {
    my ($x, $y) = @_;
    while ($x <= $#words && $y <= $#words) {
        return $c if $c = $words[$x++] cmp $words[$y++];
    }
    return $x <=> $y;
}

__DATA__
Section. 1.
All legislative Powers herein granted shall be vested in a Congress of
+ the United States, which shall consist of a Senate and House of Repr
+esentatives.
Section. 2.
The House of Representatives shall be composed of Members chosen every
+ second Year by the People of the several States, and the Electors in
+ each State shall have the Qualifications requisite for Electors of t
+he most numerous Branch of the State Legislature.
No Person shall be a Representative who shall not have attained to the
+ Age of twenty five Years, and been seven Years a Citizen of the Unit
+ed States, and who shall not, when elected, be an Inhabitant of that 
+State in which he shall be chosen.
[download]

I suppose I could write this to use a linear counting method of some sort, but that's for another day.

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