my @nlist     = ( 2,4,5,6,7 );
my $tolerance = 1;

We would like to have:

A => [2]
B => [4 5]
C => [4 5 6]
D => [5 6 7]
E => [6 7]

##</code><code>##

#my @nlist = (0,0,1,2,3,3,4,5,6,8,8,10);
#my @nlist = (0,1,2,3,4,5,6,8,10);
my @nlist = (2,4,5,6,7);

my @key_list = ('A'..'Z');
my $tolerance = 1;
my %hoa;

my %uniq;

# Check if first element has a neighbour
if ( felem_has_nbr( $nlist[0], $nlist[1],$tolerance ) == 1 ) {
    @uniq{ @nlist[ 1 .. $#nlist ] } = ();

}
else {
    @uniq{ @nlist[ 0 .. $#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;
}


sub felem_has_nbr {

    my ( $felem, $sec_in_arr, $tl ) = @_;
    abs( $felem - $sec_in_arr ) <= $tl ? return 1 : return 0;

}