@arr = qw ( up down in out left right );  # 6 elements
@sel = ( 1, 3, 4 );

$mask = 0b000000; # Need to set this to have as 
                  # many bits as the size of @arr
                  # Maybe better to pass it in 
                  # by reference?


printf("      org \$mask: %0b\n", $mask);
@revsel = reverse(@sel);
foreach $item (@revsel) {
   vec($mask, $item, 1) = 1;
   printf("\$item: $item  \$mask: %0b\n", $mask);
}
printf("    final \$mask: %0b\n", $mask);