sub test{ 
    my( $p1, $p2, $p3) = map{ scalar @_ ? shift : $_ }( 1, 2, 3 ); 
    print "p1:$p1; p2:$p2; p3:$p3"; 
};;

test();;
p1:1; p2:2; p3:3

test( 'a' );;
p1:a; p2:2; p3:3

test( 'a', 'b', 'c' );;
p1:a; p2:b; p3:c