my @a = qw/1 2 2.2 2.13 2.1.7 3.4a 10.4a.3b.8/;
my @sortables = map { [grep /[^\.]/, split/([\.a-z])/, $_] } @a;
print join "\n", map join(' ', @{$_}), @sortables;

##</code><code>##

my @list = qw/1 10.4a.3b.8 2.2 2 2.13 3.4a 2.1.7 /;

print join "\n", sort compare @list;
               
sub compare {
    my @a = grep /[^\.]/, split/([\.a-z])/, $a;
    my @b = grep /[^\.]/, split/([\.a-z])/, $b;
    {
        my ($c, $d) = (shift @a, shift @b);
        (!defined $c and  defined $d) and return -1;
        ( defined $c and !defined $d) and return 1;
        (!defined $c and !defined $d) and return 0;
        ($c > $d) and return 1;
        ($c < $d) and return -1;
        redo;
    }        
}

##</code><code>##

my @list = qw/1 10.4a.3b.8 2.2 2 2.13 3.4a 2.1.7 /;

my @cache = map { [grep /[^\.]/, split/([\.a-z])/, $_] } @list;
print join("\n", map("@$_", sort compare @cache));
               
sub compare {
    my @a = @$a;
    my @b = @$b;
    {
        my ($c, $d) = (shift @a, shift @b);
        (!defined $c and  defined $d) and return -1;
        ( defined $c and !defined $d) and return 1;
        (!defined $c and !defined $d) and return 0;
        ($c > $d) and return 1;
        ($c < $d) and return -1;
        redo;
    }        
}