Assuming that $href->{$dn}->{$mail} contains an array reference, then your choices are:

@{ $$href{dn}{mail} };
# or 
@{ $href->{dn}{mail} }; 
# or 
@{ $href->{dn}->{mail} };
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to refer to the whole array and

$$href{$dn}{mail}[0];
# or 
$href->{dn}{mail}[0];
# or
$href->{dn}->{mail}->[0]; ## yawn! :)
# or ...
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to refer to the individual elements.

However, there is a useful though slightly obscure way of simplifying the access to deeply nested structures without needing to copy data into temporary variables.

## 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.
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It's especially useful when you have to do a whole bunch of accesses to the elements of some deeply nested structureal element.