$mailbox   = 'inbox';
$subscribe = 'yes';
$country   = 'NZ';

$sel_mailbox{$mailbox}     = 'test';
$sel_subscribe{$subscribe} = 'test';
$sel_country{$country}     = 'test';

##</code><code>##

for (qw(mailbox subscribe country))
{ ${"$sel_$_{$$_}"} = 'test' }  # <= Not working!!!