Running under mod_perl, you should be able to get an Apache::Request object easily, using something like this:

# 
$c = {
  'foo.com' => {
     dbname => 'foo',
     dbuser => 'foo',
  },
  'bar.com' => {
     dbname => 'bar',
     dbuser => 'bar',
  }
};

sub param {
  my $param = shift;
  # get a request instance
  my $r = Apache::Request->instance();
  # get the hostname
  my $host = $r->hostname();
  return $c->{$host}->{$param};
}
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This obviously doesn't take into account 'www.foo.com' and 'foo.com' if they're the same thing, but a little munging of $host before using it, should allow you to find the right one, or worst case, fall back to a default.