I would like to substitute the hash name with

You don't have a hash name - which would mean you had named hashes - but hash keys in your array of anonymous hashes. If you change the key and reference that changed key, there's no value behind it - the value you are trying to acces is bound to the old (unchanged) key. To change a key, you have to copy over the value the old key is pointing at into the slot pointed at by the new key:

my $i = 1;
$varname = "_address" . $i;
$customer->[0]{$varname} = $customer->[0]{'_address'};
print "Customer Address", $customer->[0]{$varname};
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But...

$customer->[0]{_address1}
$customer->[0]{_address2}
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whenever there are keys with a number attached, that cries for a revision of the data structure: store an anonymous array as the value to that hash. Then you have

$customer->[0]{_address}[0]
$customer->[0]{_address}[1]
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and looping becomes much easier:

my $arrayref = $customer->[0]{_address};
for my $address (@$arrayref) {
    # do whatever with $address
    ...
}
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But if you really have those column names (_address1,_addresss2,...,_adressn) in your tables, then the database design is flawed. You might want to normalize the addresses out to a another table.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}