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use constant CONFIG_ITEMS => [ qw( 
  server            database             user 
...
sub help {
  die "Parameter $_[0] wasn't set in config file.";
}

use constant CONFIG_ITEMS => {
  server   => sub { my $param = shift; ...some Boolean test },
  database => sub { ... },
  user     => ...
};

use constant CONFIG_ITEMS => {
  server => qr/some pattern/,
  database => qr/some pattern/,
  ...
};

foreach my $item( keys %{CONFIG_ITEMS} ) {
  my $value = $cfg->param($item);
  help() unless CONFIG_ITEMS->{$item}->($value);
  $config{$item} = $value;
}