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

for my $i (1..$#nlist) {
    my $key = shift @key_list;
    $hoa{$key} = [grep in_range($nlist[$i], $_), @nlist ];
}

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

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