sub printUsage {
    my ($mandatory, $optional) = @_;

    print "Usage: $0\n";
    print "-$_ ", uc $_, " " for @$mandatory;
    print $/;
    print "[-$_ ", uc $_, "]" for @$optional;
    print $/;
}

##</code><code>##

my @mandatory = qw(user to_db to_sid to_site);
my @optional  = qw(password vendor type model);
my @default   = qw(from_db from_sid);

our $from_db  = "some_db";
our $from_sid = "some_sid";

my $args = {};
my $rc;
{
    no strict 'refs';
    $rc = GetOptions($args,
                     (map { "${_}=s" } @mandatory),
                     (map { "${_}:s" } @optional),
                     (map { ("${_}=s", \${"$_"}) } @default),
                    );
}

printUsage(\@mandatory, \@optional, \@default)
    unless $rc;
printUsage(\@mandatory, \@optional, \@default)
    if grep !exists $args->{$_}, @mandatory;