use CGI;

my $q= new CGI;

my Svalues = ['a', 'b', 'c'];
my $defaults = ['a','c'];
#i suppose you know how to fill these

print $q->start_form();
print $q->scrolling_list( 
                         -name=>'LIST',
                         -values=>$values,
                         -multiple=>1,
                         -defaults=>$defaults
                        );

print $q->end_form();

##you can fetch selected items with
## my @selected = $q->param('LIST');
[download]

update: i notice you asked for dropdown menu, not scrolling lists
so you should write:

print $q->popup_menu( -name=>'LIST',
                      -values=>$values,
                      -default=>'single value'
                    );
[download]

remember if you use popup_menu you can select just a single item!