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');