You might want to use the bin number as the key of the hash and add the count to it whenever you encounter it. It could look like this:

use strict;
use warnings;

my %hash;
my $data = "BIN COUNT 1 20 2 0 2 1 3 0 3 0 3 1 7 0 8 2 9 1 9 2 10 3";
$hash{$1} += $2 while $data =~ /(\d+) (\d+)/g;
print "BIN COUNT ";
print "$_ $hash{$_} " for sort {$a<=>$b} keys %hash;
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Also, please use code tags in your post to make it more readible.