#!/usr/bin/perl
 
my $string = "123123124";
my $len = 5;
 
my %substrings;
for (my $i = 0; $i + $len <= length $string; $i++)
{
        my $sub = substr($string, $i, $len);
        $substrings{$sub}++;
}
 
print "$_ => $substrings{$_}\n" for sort {
        $substrings{$b} <=> $substrings{$a}
                ||
        $a cmp $b
} keys %substrings;