my @Datum = qw( 12.4.2007 1.2.2007 31.8.2007 05.05.2005 );

@Datum = 
    map{ $_->[0] }
    sort{ $a->[3] <=> $b->[3] || 
        $a->[2] <=> $b->[2] || 
        $a->[1] <=> $b->[1] }
    map{ [$_,split /\./, $_] }@Datum;

my %seen;
@Datum =
   map{ $_->[0] }
   sort{ $a->[3] <=> $b->[3] ||
       $a->[2] <=> $b->[2] ||
       $a->[1] <=> $b->[1] }
   map{ [$_,split /\./, $_] }
   grep{ !$seen{$_}++}@Datum;

map { $h{$_}++ == 0 ? $_ : () } @_;

map { $h{$_}++ == 0 ? $_ : () } @_;

map { $h{$_}++ == 0 ? $_ : () } @_;

[$_,split /\./, $_]

[$_,split /\./, $_]

 [ $_, split(/\./, $_) ]