Benchmark: timing 2000 iterations of Krambambuli, blazar...
Krambambuli:  4 wallclock secs ( 3.28 usr +  0.01 sys =  3.29 CPU) @ 607.90/s (n=2000)
    blazar:  1 wallclock secs ( 0.80 usr +  0.00 sys =  0.80 CPU) @ 2500.00/s (n=2000)

##</code><code>##


use strict;
use warnings;
use Benchmark;
use Data::Dumper;

use constant MIN => 2;

my $str = 'aabcdabcabcecdecd';
my $min_length = 2;
my $min_count  = 2;

timethese(2000, {
    'blazar'      => sub { count1( $str ) },
    'Krambambuli' => sub { count2( $str ) },
});

{
my %count;

sub count1 {
    my( $string) = @_;
    my $length = length( $string );

    if ($length < MIN) {
        $count{$_} == 1 and delete $count{$_} for keys %count;
        return \%count;
    }

    for my $l (MIN..$length) {
        my $s = substr( $string, 0, $l );
        $count{ $s } += 1;
    }
    count1( substr( $string, 1 ) );
}
}

sub count2 {
    local $_=shift;
    my $l=length;
    my %count;

    for my $off (0..$l-1) {
        for my $len (MIN .. $l-$off) {
            my $s=substr $_, $off, $len;
            $count{ $s } ||= ()= /$s/g;
        }
    }
    $count{$_} == 1 and delete $count{$_} for keys %count;
    \%count;
}