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