my $string = (join(".", sort split(/\./, "Pugh.Barney.Pugh.Barney.McGrew.Cuthbert.Dibble.Grub")));
my @array = ($string =~ m/(\S+)\.\1/g); # Capture duplicate
$string =~ s/(\S+)\.\1//g;    # Eliminate duplicates
$string =~ s/\./\n/g;         # Change \. to \n
foreach (@array) {            # Print duplicates
    print "\n$_.$_";
}
print "$string";              # Print the rest of the list

##</code><code>##

    $string =~ s/(\S+)\.\1/'$1.$1'/g;    # Add quotes

and plugged into merlyn's reply

$_ = join(".", sort split(/\./, "Pugh.Barney.Pugh.Barney.McGrew.Cuthbert.Dibble.Grub"));
$_ =~ s/(\S+)\.\1/'$1.$1'/g;	  # Add quotes
my @keepers = grep defined $_, /'(.*?)'|([^.]+)/g;
print map "$_\n", @keepers;