# set up default values for parameters:
my %pars = (
    item => 'default value',
);    
GetOptions (
    \%pars,
     'items=s', # NOTE MISPELLING-- "items" s/b "item"
) or die "bad GetOptions $!";

# use $pars{item}, which is unaffected by an --items argument

##</code><code>##

my @opts = (
    { name => 'item1', type => '=s', dval => 'default_value1' },
    { name => 'item2', type => '=s', dval => 'default_value2' },
);

my %pars = map { $opts[$_]{name} , $opts[$_]{dval} } 0..$#opts;
my @args = map { $opts[$_]{name} . $opts[$_]{type} } 0..$#opts;

GetOptions (
    \%pars,
     @args
) or die "bad GetOptions $!";