use warnings;
use strict;

my @nlist = (0,0,1,2,3,3,4,5,6,8,8,10);
my @key_list = ('A'..'Z');
my $tolerance = 1;
my %hoa;

my %uniq;
@uniq{@nlist[1..$#nlist]} = ();

for my $centroid (sort {$a <=> $b} keys %uniq) {
    my $key = shift @key_list;
    $hoa{$key} = [grep in_range($centroid, $_), @nlist ];
}

print "$_ => [@{$hoa{$_}}]\n" for sort keys %hoa;

sub in_range
{
    my ($centroid, $testnum) = @_;
    return abs($centroid - $testnum) <= $tolerance;
}