@{ $$href{dn}{mail} };
# or 
@{ $href->{dn}{mail} }; 
# or 
@{ $href->{dn}->{mail} }; 

##</code><code>##

$$href{$dn}{mail}[0];
# or 
$href->{dn}{mail}[0];
# or
$href->{dn}->{mail}->[0]; ## yawn! :)
# or ...

##</code><code>##

## Make the localised glob *mail 
## act as an alias to the nested array

{
    local *mail = $ref->{dn}{mail}; 
    print @mail; ## Print the whole array
    print $mail[0]; ## The first element
} 
## *mail reverts to it's old value here.