What you want is some Histogram. Have a look at CPAN. Otherwise, use a hash for counting:

use strict;
use warnings;

my @array=qw ( 1 2 3 10 11 12 13 20 21);
my $bin = 10;
my %hist;

map {    $hist{ (int(($_-1)/$bin)+1)*$bin }++ } @array;
print join ",", map { ($_-9)."-$_ $hist{$_}" } sort keys %hist;
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